Local mods are created in the RimWorld/Mods folder, and can be manually deleted, or deleted by clicking the corresponding button underneath the mod details.
Doing this will make sure any updates to the steam version will not break your game in progress. Local mod copiesYou can make a local copy of any steam mods (or all active steam mods at once) by clicking the corresponding button underneath the mod details (or active mods list).
See the documentation for details on how to implement this.