To REMOVE the existing mail account:
Step 1 of 5
From the home screen, tap Settings.
Step 2 of 5
Tap Mail, Contacts, Calendars.
Step 3 of 5
Tap the Exchange or other email account to remove.
Step 4 of 5
Scroll down and tap Delete Account.
Step 5 of 5
Tap Delete from My iPhone to confirm. Note that other types of iOS devices will display similar confirmation notices.