One idea is to save your farm save file to another source temporarily such as iCloud or an external drive, and then delete and reinstall the game. If you delete the game before saving your farm save file, it will delete it along with the game.
You can find your farm save file in the Files app > On My iPhone (or On My iPad) > Stardew Valley. It's important to confirm your farm save file has been saved to another location before deleting and reinstalling the game.
If you need more details or help doing this before you delete the game, just let us know.
It might be helpful too to know which version of Stardew Valley you're running. Is it 1.5.6.39? You can find the version number by tapping on the question mark icon on the loading screen.