My boss signed in on my computer, and now it always defaults back to his account on the forum. If I click the avatar > logout, then go back and hit login again it just signs me right back into this account - no chance to change the user name.
Clear your cache/history, restart browser and try again.
While we are on topic tho I have had an opposite experience here (although probably related). Sometimes I log in and it sucessfully logs me in but then I’m not really logged in. No K circle in the upper right, I cannot post, etc. Usually I try 3-4 times. Sometimes it works by the last try and other times I have to clear cache/history and try again.