In Xbox 360 storage, move your Gamertag profile (usually the first item) to the USB drive. You only need to do this once—Horizon will read the profile ID from the USB.
Click the "Rehash & Resign" button (looks like a refresh symbol). Horizon will overwrite the IDs with those from your USB drive’s profile. Then click "Save to Device" – choose your USB path and overwrite the existing save. horizon app xbox 360
Download a savegame.dat or Save.sav from a trusted source (e.g., Nexus Mods’ Xbox 360 section). In Horizon, click "File" → "Open" → select the downloaded save. Horizon will show details: Console ID, Profile ID, and Title ID. In Xbox 360 storage, move your Gamertag profile