change old password
How to change the old password in the backup section.
Windows, Windows 10
Question marked as
Well, there's no way to change an exixting encrypted backup password.
If you're running iOS 11 or later, you can create a new encrypted backup (using a different password) of your device. See section "If you can’t remember the password for your encrypted backup" in this support article:--> About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support
Regards
Giulio