If you are referring to the Mail app...go to Settings > Mail, Contacts, Calendar > Select the appropriate account then select delete.
Edit: if you are referring to changing your iCloud email...go to Settings > iCloud > delete account then it gives you an option to create a new one. FYI...by doing this, you will not have access to anything you saved to the iCloud account you deleted because you can't be signed into 2 accounts at once.