Open your email then open Settings via the dropdown. Select your account from there and scroll down to General Settings. You should be able to change your signature from there. ,
You can change your signature by going to Settings --> Accounts ---> General Settings ---> Signature. Tap on the Signature and you can change it there.
I just ordered the Case-Mate Tough Case. There really aren't a lot of options out there yet for the Z3C and this seemed to offer the best protection, although it doesn't cover all the ports. We'll see how it goes.