Time Machine

Refer to link and excerpt from that same line below

About the Time Machine backup password on Mac

To encrypt the Time Machine backup, you must provide a backup password.

No one but you should have access to your password.

Choose a hint that helps you remember your password. To restore files from encrypted Time Machine backups, you must provide your backup password.

Important: If you have iCloud Keychain enabled, you may be able to retrieve your backup password through Keychain Access. Otherwise, you can’t restore or recover your data if you lose or forget your password.

If you have not done what was explained in above and in case it was over looked

Choose a hint that helps you remember your password. To restore files from encrypted Time Machine backups, you must provide your backup password.

Important: If you have iCloud Keychain enabled, you may be able to retrieve your backup password through Keychain Access. Otherwise, you can’t restore or recover your data if you lose or forget your password.

I see no mention of Resetting the TM Backup Password

The only, sadly so, option would the Reformat the Entire TM Backup and start again

Erase and reformat a storage device in Disk Utility on Mac - Apple Support (CA)

Understand - this will Remove ALL DATA from the Drive.