Go to your name, upper left, click on "User Control Panel", click on Edit Signature.

 

You ask a question in Personal Message but interesting enough, I can't return your answer do to your PM settings on no PM.

I thought the system overrode it for friends and if you set someone a message and allow them to at minimum return it. Guess not.
Basically, near top, vertical scroll. Hit User CD. Then down left vertical bar you will see Edit Avitar and Edit signature.

 

Welcome To The HD Forum From Michigan, The Home Of Hell And The Greatest Of The Great Lake States :>)

 

Welcome to HD Forums.