How do I change the name of a custom tab?

You can edit a custom tab name any time you want, by going back in to the "Settings" tab, "Custom tabs" and clicking on the edit icon next to the tab name you want to change.

Note that the "About me" section of your profile is not a separate tab (you could make a tab dedicated to "about me"-related material, of course). While the content is customizable, the name of the "About me" section is not.