how can i retrieve my forgotten password for encrypted backup

You can't. You have to restore as new and delete the encrypted backup. From the Apple in article....

You can't use an encrypted backup if you don't have the password. If you forgot your password, the only way to turn off backup encryption on your device is to erase your device and set up as new. Erasing removes all data from your device. If you don't want to erase your device, restore from an iCloud backup instead.

RoeLaff wrote:

How can I retrieve my forgotten password for encrypted backup

You can't retrieve it, but you have unlimited guesses. If you want to try some uncheck Encrypt Backup and you will be prompted immediately. If you are like most people it is a passcode that you have used in the past. The passcode entered the very first time you created an encrypted backup never changes, so go back in time.