On the green bar is a "UserCP" selection that gets you access to your signature and avatar edits. You can change the title text to be what you want (within reason and the rules of decorum).
If you are having trouble, a moderator can do it for you.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW