I got stuck looking for the signature on Outlook 2010 and since i found it , this is how you do it :

1. Click on the File tab.
2. Click Options.
3. Click Mail.
4. In the Compose messages section at the very top, click the Signatures button.
5. Either select Delete, New or just highlight a signature and edit.
6. When you are finished, click OK until you are out.