Is this an encrypted backup for a device running iOS 11? If so, you can reset your Encryption password without having to erase your phone:
You can’t restore an encrypted backup without its password. With iOS 11 or later, you can make a new encrypted backup of your device by resetting the password. Here’s what to do:
- On your iOS device, go to Settings > General > Reset.
- Tap Reset All Settings and enter your iOS passcode.
- Follow the steps to reset your settings. This won't affect your user data or passwords, but it will reset settings like display brightness, Home screen layout, and wallpaper. It also removes your encrypted backup password.
- Connect your device to iTunes again and create a new encrypted backup.
You won't be able to use previous encrypted backups, but you can back up your current data using iTunes and setting a new backup password.
To turn off iTunes backup encryption, uncheck the Encrypted backup box in iTunes and enter the password.
About encrypted backups in iTunes - Apple Support
If you back up to iCloud, your backup is automatically encrypted and you don't need a passcode to use it.
Best,
GB