iOS does not allow downgrading official App Store apps. The only way to play Granny 1.8.1 on iOS is:
The beauty of Granny version 1.8.1 lies in its variety of escape methods. While later updates added the Boat and the helicopter, 1.8.1 focused on the "Big Three." Mastering these was essential for anyone claiming to be a pro player. granny version 1.8.1
If you encounter a legacy project using Granny 1.8.1, ensure you are running the matching exporter toolchain (e.g., 3ds Max 8 or 9 on Windows XP). Modern OSes may require compatibility mode settings. iOS does not allow downgrading official App Store apps