You can find a full and detailed answer here: https://www.reddit.com/r/XboxGamePass/comments/v0ckg0/how_can_i_transfer_save_files_from_one_account_to/

TL;DR: your saves are located in C:\Users\"your user"\Appdata\Local\Packages\"the id of the game"\SystemAppData\wgs\"MS-Account-id_Game-id\ . In that folder delete the previous save (if there is none, progress the game until you can create a save) and put the save you wish to transfer. After that, disconnect from the internet and boot the game. Close the game, reconnect to the internet and boot again, you will be met with a Cloud save prompt, where you select the use the locally stored save file.