Q: Restore encrypted backup

You can't.

If you don't remember the password, the backup is completely worthless.

The only thing that can be done with it is to delete it.

The backup will the useless but the worse part is that all future backups will be encrypted as well with the same PW. You can either follow the instructions here:

https://support.apple.com/en-us/HT205220

Or use the time consuming workaround below to prevent having to erase your phone.

To bypass the issue of encrypted backups always being encrypted:

Backup to iCloud. Then erase the phone. Then backup to iTunes as an unencrypted backup. Then restore from the iCloud backup. Then backup to iTunes again.

From: https://discussions.apple.com/message/30209896?tstart=0#30209896