It's easy, and will not impair any save files or your ability to start new games. Here, of course, the hope is that it will restore your ability to start new games.
Shut down the game, fire up Steam and go to your Steam library. Right click Civ VI and choose Properties from the popup menu. In the Properties window, choose the third tab (Local Files) and then choose the last item (Verify Integrity of Game Cache...). It will scan your installation for missing or corrupted files and download any necessary replacement files. If it does download any files, I would recommend re-running Verify Integrity until no more files are downloaded.
This may or may not fix your problem, but it's usually the first thing to try when you see glitchy behavior in the game, so it's worth learning how to do this.