Modifying your Outlook Signature is simple:

1. Open Outlook.
2. Go to the Tools Menu.
3. Select Options.
4. Click on the Mail Format tab.
5. Click on the Signatures button at the bottom.
6. Your email signature should already be selected (it's probably called default), if not then go ahead and select it.
7. Click on Edit.
8. Make the appropriate changes.
9. Click Ok.
10. Make sure in the Mail Format tab, above the Signature button, your signature is selected for both New Messages, and Replies & Forwards.
11. Click OK.

And you are done