Whenever I install a new Lazarus version I always do this as a "secondary installation" and use a new directory for the Lazarus user settings. This way the new installation can coexist together with the old one, and I always can switch back if something goes wrong.
I think it is the second page of the installer where you can check the "secondary installation" box. Later you are asked about a folder for the config settings - this should be a new directory without any old config files (I name it c:\laz-configs\laz<version>_fpc<version>). Do not register any files with Windows to open on a double-click, because with multiple installations there is a high chance that the registered version will open the wrong IDE.
To fix your current mess, I agree that it is probably the fastest option when you really delete the old Lazarus folder and its config folder (by default it is in c:\users\<your name>\appdata\local\Lazarus), and reinstall a-new.