Q: I forgot the encrypted password for my back-up, how can i change the password??

If you don't have the password or you forgot, Apple is very specific:

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

See more here:

About encrypted backups in iTunes - Apple Support

You cannot reset it.  If you cannot remember the password for that backup, you will never be able to restore it to a new device.  If you use a Mac, check your keychain to see if the password was stored there.

You delete the old backup and create a new one because without the password the old backup is irrecoverable.

How do I delete the old back up?  If the back up is deleted, and a new is created, can you turn off the encrypt?