Need to change Engine Service account back to Network Service

I mistakenly configured SQL Server to run under an AD account. Now, I need to set it to run as Network Service but it prompts for me to enter the password (which I don't have).
any ideas?
Thanks

SOLVED: All I had to do was to delete the passwords in the password boxes, then click OK. The sql server account now shows Network Service correctly.

Any specific reason for you to not use AD account?

I first had it as an AD account. The app vendor requires a local service.

You probably should be using the virtual accounts instead of network service. That is the way it would be defined for a default installation and is the recommended accounts.