When you go to twitter.com in Safari on the iPad it redirects you to the mobile (tablet) version of the site. This site does not give you access to your settings, which is where your email address can be changed.
Unlike some mobile sites, Twitter does not seem to offer a link back to the desktop version.
But all is not lost. You can get a third party browser like Atomic or iCab Mobile. These browsers can be set to 'lie' and identify themselves as a desktop browser. If you do that you can go to the desktop version of the Twitter site and edit your settings.
Sometimes this doesn't work, because the desktop sites are not fully compatible with mobile browsers; however I tried this with Atomic and The twitter.com site seems to work properly, at least well enough to go to settings and make changes.