If you mean you took the exam and failed, and so didn't see the party, I'm afraid there's no way to redo it beyond loading an earlier save, sorry 😔
As for the other question, as long as you don't delete the app you'll keep your save. When you download a new version and run it, you'll be prompted to update rather than install. Once that's done, you can open the game and all your saves will be there 😊