Based on your description I'm fairly certain that it's not your address book that needs tweaking, but the display name associated with your e-mail account in Outlook.
I'm basing the following on that presumption. To shorthand this if I list something like ALT+F, I, A that means hit ALT+F, followed by I, followed by A.
1. Open Outlook and hit ALT+F, I, A which will put you in the Account Information Pane.
2. In Outlook 2010 it makes no difference which account you choose from the dropdown before the next step, so you'll have to choose it again anyway. So Tab to the Account Settings button and activate it, at which point you have to activate the dropdown for account settings (don't ask me why).
3. This brings up the account settings dialog. What follows presumes you have more than one e-mail account set up in Outlook. You'll be able to omit a step or two if you have just one.
4. The e-mail tab is open, which is what you want to deal with. If you have more than one account, you will want to hit TAB six times until you land in the accounts list, then down arrow through the accounts until you get to the one for which you wish to make a change and select it. You can skip this step entirely if you have only a single account.
5. Hit ALT+A, which at this point invokes the Change dialog. You should immediately land in the edit box for "Your Name" which is what you want to change so that whatever name you prefer is used as the prefix on your e-mail address in outgoing e-mail messages. If by some chance you aren't sitting in that box then hit ALT+Y to throw yourself into it. Edit as desired.
6. (Optional) Check out any of the other account settings you might want to tweak.
7. Hit ALT+N to activate the next button, this will do a test of your account settings, which should succeed if the only thing you changed is your name.
8. After the test confirms that everything is OK, hit ALT+C to close the test dialog.
9. Activate the Finish button to close out the process.
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall