In the blue bar at the top, you'll see "User CP" (User Control Panel)... click on that, then under "Settings & Options" click on "Edit Signature". Then edit away.
That's about it. Good luck.
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2011 Keystone Alpine 3500RE w/ Trail Air Tri-Glide
2011 GMC Sierra Denali 3500HD SRW B&W Turnover Ball w/ 5th Wheel Companion