Follow:
AdminCP -> Styles & Templates -> Style Manager
Then for the style you are using, select "Edit Templates" from the "Template Options" and then scroll down and double-click on the "header" template.
Locate the code:
HTML Code:
<li class="welcomelink">{vb:rawphrase welcome_x_link_y, {vb:raw bbuserinfo.username}, {vb:link member, {vb:raw bbuserinfo}}}</li>
and change this to:
HTML Code:
<li class="welcomelink">{vb:rawphrase welcome_x_link_y, {vb:raw bbuserinfo.musername}, {vb:link member, {vb:raw bbuserinfo}}}</li>