The first thing you should do is find and backup any existing projects, although it sounds like you may not have any to worry about if you did all your work in another program and are going to be importing. All the same, search for “.scriv” using the Windows search tool and see if it brings up any project folders that you want to keep–projects that you created when working in an earlier version of Scrivener, e.g. “MyGreatNovel.scriv”. These are probably all stored in your Documents folder, but by viewing the full list of search results you can check the file paths to see where they are, e.g. C:\Users\YOURUSERNAME\Documents\MyGreatNovel.scriv. All you care about here are folders with the .scriv extension.
The main thing here, besides finding out where your projects are in general, is to ensure you did not somehow save them inside the Scrivener program directory. So if you find them in, say, C:\Program Files (x86)\Scrivener\ then move them from there to your desktop or your documents folder or somewhere else not inside that Scrivener directory. (This is the directory where you’ll also find Scrivener.exe and uninstall.exe; it was created when you ran the installer.)
If you find projects you want to hang on to, you can navigate to them in Windows Explorer, right-click each .scriv folder and choose “Send to \ Compressed (zipped) folder” to make a backup. So long as they’re not in that installation directory, they won’t actually be affected by the uninstallation process, but it’s always a good idea to make backups, as you know!
Now to uninstall Scrivener, you can either navigate to the Scrivener installation directory (e.g. C:\Program Files (x86)\Scrivener) and run the uninstall.exe or use the Windows tools to do this by opening Control Panel and going to Programs and Features, then finding Scrivener in the list and choosing “Uninstall” from the menu above the list. To be extra thorough, you can go to the Scrivener installation folder in Windows Explorer and manually delete it if it wasn’t entirely removed by the uninstaller. (If you only ever had the NaNoWriMo version, unless you downloaded a foreign language dictionary, this should be removed automatically, and you really don’t need to worry about it in any case–this is really more an issue for beta testers who used earlier versions.)
Then download the latest version of Scrivener (1.0.3) from here. Choose to save that file, then navigate to the download folder in Windows Explorer and double-click the Scrivener-installer.exe to run it.
Once you have Scrivener installed, you can open any of your existing projects using File > Open or the “Open Existing Project…” button in the New Project window.
When you create a new project, you’ll choose a name and a location to save it. Scrivener creates the project’s .scriv folder, and that is where your project is saved forevermore. Scrivener auto-saves your work every two seconds of inactivity (you can adjust the time in the General tab of Tools > Options…) and when the project is closed; this is the same process as occurs when you choose File > Save. This just overwrites the files in the location they’re at, the same as choosing “Save” in Open Office just overwrites the file you’re working on. If you choose “Save As”, you’re creating a copy of the entire project and a new .scriv folder is created at the location and with the name that you choose at that point, and you then are working in that new version of the project–again, the same as choosing “Save As” in Open Office.
If you want to save a backup of your project–say you reach a point that you want to save and then be able to revert to if you don’t like the changes you make after this–use the File > Back Up > Back Up To… option. When you do this you’ll again make a complete copy of the project, and you can choose to make it a zipped copy (convenient as it makes you be more intentional about opening the backup, thus helping prevent accidentally working in a backup instead of a current version) and to date it as well, in addition to choosing the name and location. Unlike “Save As”, this won’t change the project you’re currently working in–the backup is made and saved, but you’re still in the original project, so saves are still being made to that original location. This helps prevent having a trail of “active” projects on your computer, so it’s not as likely you’ll accidentally start working in the wrong version of the project and end up with two different versions with edits you need to merge.
If you want to change the location of a project, just close it in Scrivener and then move the project’s entire .scriv folder to the new location in Windows Explorer. You’ll need to use File > Open to access it once at that point (or open the .scriv folder in Explorer and double-click the project.scrivx file to open it) since the “Recent Projects” menu won’t be able to find it after you move it, but once you’ve opened it, the project will again appear in the “Recent Projects” list.
I hope that helps explain things. Once you’ve got Scrivener installed, I recommend starting the Interactive Tutorial (available from the “Getting Started” category in the New Project window or via Help > Interactive Tutorial) as that’s an actual Scrivener project that will walk you through the basics and then some of how the program works and should give you a good handle on things. It will take a little while, so I’m guessing you haven’t had time to do it already since NaNoWriMo sucks “time” right out the window, but you can do it in a few sittings and it’s a good way to get familiar with the program.