How do I unencrypt my iphone backup if i dont know the password

The only thing you can do (if you still have the data on your iphone, and it is running iOS 11 or later) is to make a new encrypted backup with a different password.

Read this Support guide:

About encrypted backups in iTunes - Apple Support

Regards

Giulio

You don't. The best you can do is turn off encryption for future backups by following https://support.apple.com/en-us/HT205220 if your iPhone is running iOS 11.

You're welcome