Create the copy before you need it
Open My List and choose Create backup. Copy all the text from the backup box into a note or document you control. The copy holds saved title references and manual checkpoints, not video files, passwords or subscription details.
Merge in the destination browser
Open My List in the browser where you want the choices. Paste the complete backup into Restore backup and choose Merge backup. Existing checkpoints are kept. Valid saved titles and additional checkpoints are added without erasing what is already there.
Read the result
The status message reports additions and skipped entries. Unknown titles or episode numbers outside the recorded season are not imported as if they were valid. A malformed text file should not be treated as a blank list; the tool reports a problem instead.
This is a manual backup, not automatic syncing. Make another copy after important list changes. If you are preparing offline viewing too, follow the separate download guide for the selected service's media files.