it's kind of stupid - you need to have a standard android "menu/settings" button, which on the ouya is only available as single-click on the ouya controller center button.
so, click into the google account, press the controller center black button once, and it will pull up the delete/change/etc. menu.
once you delete the account, a couple ways to add a new one is simply open up play store or gmail or google voice.
(Iately I find this more and more of an annoyance with certain apps that have no other way to pull up that function. there are ways to map that onto usb keyboard, which are not too difficult with the keylayout files, I think I really need to get around to setting that up)