hi
what version xl are you using????
2003> on the menu bar....Tools>options>general tab>user name
2007>office button>excel options>not sure after this.
After you sign in, the sign in button changes to a sign out button. Just to
the left of the sign out button is a "help" link and just to the left of
that is a tiny icon to edit your profile where you can change your display
name.