Email signatures are a useful tool, and the “sent from my iPad” message has become iconic. But if you don’t want to use Apple’s default signature, it’s easy to change.
Just head to the Settings app, then click on “Mail, Contacts and Calendars” on the left side of the screen. Down the middle of the main screen, you should see an option for “signature”.
Click there, and replace your signature with whatever you’d like.