Overall I would recommend to before doing anything to make a copy of your project folder and if possible a backup too.
It sounds though that at some point you just opened your project with AGS 3.5.1.
If that is the case, there should be a Game.agf.user file in the directory and maybe a few other ".user" files there, they could be removed.
If you absolutely didn't, my other guess would be that in your AppData/AGS in Windows there is a preference file written by AGS 3.5.1 there, so you could remove that or reinstall the 3.5.0 version you want - I don't remember if 3.5.0 and 3.5.1 attempt to use the same install dir or not.
Any reason to not use 3.5.1 or the most recent 3.6.0 version or the beta 3.6.1?
Also, I really recommend using some form of version control, this would help identifying which files changed when.
It sounds though that at some point you just opened your project with AGS 3.5.1.
If that is the case, there should be a Game.agf.user file in the directory and maybe a few other ".user" files there, they could be removed.
If you absolutely didn't, my other guess would be that in your AppData/AGS in Windows there is a preference file written by AGS 3.5.1 there, so you could remove that or reinstall the 3.5.0 version you want - I don't remember if 3.5.0 and 3.5.1 attempt to use the same install dir or not.
Any reason to not use 3.5.1 or the most recent 3.6.0 version or the beta 3.6.1?
Also, I really recommend using some form of version control, this would help identifying which files changed when.