I just reinstalled. Also, how can I install GL?
1). Go here:
https://forums.terraria.org/index.php?threads/game-launcher-3-2-1-5.1061/
2). Download and install it.
3). GameLauncher will rename the original Terraria as something like TerrariaBackup.exe or something like that.
4). Go here:
https://forums.terraria.org/index.php?threads/download-and-installation-instructions.13418/
5). Scroll down to the instructions ("How To Install STW" button)
6). Follow Steps 1-3.
7). Step 4 says to rename Terraria.exe .... don't do this. Leave that file as-is (it's the Game Launcher prog).
8). When you copy STW's version of Terraria.exe, the computer will ask if you wish to overwrite. Say No, and you will get a file called "Terraria (1).exe".
9). Rename this to something like Terraria_STW.exe
10). Follow the rest of STW's instructions.
11). Open GameLauncher (or just launch Terraria from Steam which should open GL).
12). Click the "Add App" button.
13). Point it to Terraria_STW.exe.
14). Call it "Terraria STW" or something like that.
Now, it should already have Vanilla Terraria in its list, and now it will have Terraria STW. You can also download tModLoader (not required or needed for STW, mind you) if you wished for Modded Terraria (just remember tModloader's installer will rename GameLauncher's Terraria.exe, you'll have to manually fix it), you'd simply add that to the list as well, and you can also do this for NTerraria and other similar standalones.
Either way, you can launch whichever one you want, anytime you want, without having to revert or move files around.
EDIT: a Caveat: When Terraria 1.3.6 or whatever the next update is comes out, Steam will automatically overwrite GameLauncher's .exe. You might want to make a backup copy of this .exe and put it somewhere else.
When that happens, you would...
1). Rename Terraria.exe to Terraria_Vanilla.exe (or something like this)
2). Copy and Paste your backup Game Launcher Terraria.exe back into its proper place.