How do I change my default email address?

Click the Settings link.

In the Ways to Contact sidebar, under Email Addresses, if you have added multiple emails, you can choose the one you want as your default.
You can tell what email address is your default by the star icon next to the email address.

When you hover by the other email address, you will see a faint star icon. If you want to change your default email address to that one, click the Faint Star icon and it will make your default email address change.