HI gwebb -
Click on the User CP link on the main link bar.
If you have "Edit Avatar" and "Edit Signature" listed in the "Edit Settings" box on the left side of the screen, then you have sufficient member priviledges to update those settings.
If you do not see those setting options, then your level of membership does not provide you with the priviledge of customizing those settings.
best regards -
mqqn