You cannot change your screen name by yourself. You must contact a moderator - they can change your screen name. Look at the bottom of the portal page, see which moderators are online (shown in bold), contact one of them. I think they'll be able to assist.
All your posts and thread will remain in place and simply show the new screen name.
Lucius and Danielle
1992 29' Excella Classic / 2010 Interstate
2005 Chevrolet Suburban K2500 8.1L
2018 GMC Sierra K1500 SLT, 6.2L, Max Trailering
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...