Changing Document Owner
To change the document owner, click the User of the document as shown in the screenshot below.
Once you change the document owner in the ensuing popup, make sure you click the "Update Document" button to save it.
Note: if the user isn't added to the folder the document resides in, then that user may not appear in the Document owner dropdown. To add a user to the folder, select the folder and click the pencil icon next to it. Then under "invite users", select user(s) and click "update folder" button.