How to change password for encrypted backup if old password is forgotten

Sorry, but you can't. See the following from About encrypted backups in iTunes - Apple Support

There is no way to recover your information or turn off Encrypt Backup if you lose or forget the password.

and

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

You can't use an encrypted backup without its password.

When you try to use a backup, you might see a message that says, "The password you entered to unlock your backup was incorrect." If so, one of these things happened in the past:

• You turned on the iTunes encryption option.
• Whoever set up your device or transferred your data to a new device turned on the iTunes encryption option.
• Someone set up an installed profile on your device (often for business or education). If so, contact your system administrator.

If you don't have your password, here's what to do:

• If someone else set up your device, ask them for the password.
• Use an iCloud backup instead of iTunes. If you don't have an iCloud backup, you can make one.
• If it's still relevant, try using an older iTunes backup.

Turn off iTunes backup encryption

To turn off iTunes backup encryption, uncheck the Encrypted backup box in iTunes and enter the password. If you can't remember your password, you have two options:

• If you have an iCloud account, you can use iCloud to backup and restore instead of using iTunes.
• If you don't have an iCloud account, the only way to turn off encrypted backups is to erase your device and set it up as new.