I don’t know if I put this in the right category but still…
My friend gave me a copy of a game I never heard of and I don’t know if it’s copyrighted so if I take like the gun system and change it,
-Do I have to change everything about it?
-Do I have to do something?
-Or is it something I can’t do?
If the game isn’t opensource you’ll need permission from the original owner to use it, otherwise you technically don’t have access to use the content yourself.
Changing one small part of the game won’t help you avoid this since the game will still feature assets which you may not be allowed to possess and use.
If the game was stolen and you’re reported for using its assets without permission, Roblox are completely entitled to take action.