Reset Encrypted Backup password

Try these steps:

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

1. On your device, go to Settings > General > Transfer or Reset [Device], then tap Reset.
2. Tap Reset All Settings and enter your device passcode.
3. Tap Reset All Settings.
4. 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.
5. Connect your device to the Finder or iTunes again and create a new encrypted backup.

You won't be able to use previous encrypted backups, but you can use the Finder or iTunes to back up your current data and set a new backup password.

If you have a device with iOS 10 or earlier, you can't reset the password. In this case, try the following:

• If someone else set up your device, ask them for the password.
• Use an iCloud backup instead. If you don't have an iCloud backup, you can make one.
• Use an older iTunes or Finder backup.

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