forgot backup password

See Disable iOS backup encryption - Apple Community. In particular:

If you have forgotten the password, or were not aware that encryption had been enabled, consider that it might be any computer, Apple ID, Wi-Fi password or device PIN that you've used in the past, working from the assumption that you might have reused a common password that you were using when encryption was first activated. See About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support for details.

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From: If you can’t remember the password for your encrypted backup

If you can’t remember the password for your encrypted backup - Apple Support

"You can’t restore an encrypted backup without its password. With iOS 11 or later or iPadOS, you can make a new encrypted backup of your device by resetting the password."