You change your signature in xzewd with the Signature dialog. You get the Signature dialog by selecting "Signature..." from the Sender menu.
The Signature dialog appears in two forms, one when the Use Command button is on and one when it is off. Clicking the Use Command button will toggle between the two forms.
Here's a sample Signature dialog, with Use Command off. Click something to find out more about it.
Here's a sample Signature dialog with Use Command on. Click something to find out more about it.