It seems there is no way to set defaults through the mail app itself but if you go into the sync settings (by clicking on the green sync circle next to account name) and then hit the 3 dot icon you get the option to remove the account.....at least on the Verizon version...YMMV..
Is it me or the new email app just terrible? There doesn't seem to be a way to search or attach files that aren't pictures or videos. Am I missing something?