What game version are you running? Ensure you download the same version of Simple Trainer (trainerv.asi) as your game version.
You can see the upload history on the Simple Trainer mod page.
In order to play an older game version (Non-Cracked GTA V) you will need to replace the following files:
A quick search of older game version you wish to use can be found on Google or YouTube, for example:
"GTA V downgrade to 2245"
You will then need to "copy update.rpf / common.rpf to mods folder" because you cannot have a version mismatch between the game update.rpf / common.rpf and mods .rpf files.
To prevent the game from auto-updating and reverting all of the work you have just carried out above, you will need to install NoGTAVLauncher.
Unless the author of the modifications you have decided to install created a .OIV uninstaller then no, unfortunately you will need to uninstall manually.
You can find which files a .OIV installs by changing the .OIV file extension to .ZIP, open it with 7zip / WinRAR and open the assembly.xml file with a text editor such as Notepad / Notepad++.
You should now see all the paths the files are being placed into via the .OIV installer, now open OpenIV go inside the mods folder and delete the respective files accordingly.
Of course this can be a very time-consuming job if the installer contains many lines and therefore it may be easier just to wipe the mods folder and start again.
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