This procedure is applicable for iOS devices (iPhone, iPad, etc.) running iOS 6 or higher. The screenshots shown below were taken from an iPhone 6 running iOS 8.0.2. Other devices will be similar.
To edit an Exchange email account on an iOS device, perform the following:
1. From the home screen, tap Settings.
2. Tap Mail, Contacts, Calendars.
3. Tap the Exchange account.
4. Tap the email address next to Account.
5. Update the account settings as needed and tap Done.