Here is the link to directly change (you need to be logged in!)
Here’s a little known fact:
Even though we have had @googlemail.com addresses until now, “secretly” we’ve also had @gmail.com addresses, which were simply not shown. If you have the account firstname.lastname@example.org and someone sent the mail to email@example.com you would still get it.
Didn’t know that? Why don’t you try sending a mail to yourself with the alternate address? That includes you @gmail.com users! Prefer @googlemail.com? Then use it! Simply set up a send mail as in settings > accounts, follow the on-screen instructions and you can choose which email address you want to send from. It really is that easy.