This procedure is applicable for iOS devices (iPhone, iPad, etc.) running iOS 6 or higher. The screenshots included below were taken from an iPhone 6 running iOS 9.1. Other devices will be similar.
Prior to starting this procedure, obtain your account-specific information from the Customer Portal according to the following article:
How to Obtain POP or IMAP Account Information from the Customer Portal
To edit or update an existing email account with IMAP settings on an iPhone, iPad, or other iOS device take the following steps:
1. From the Home screen, tap Settings.
2. Tap Passwords & Accounts.
3. Tap the description of the Mail account to update.
4. Next to Account, tap the email address.
5. Update the account-specific Email settings obtained from the article linked above.
a. Tap Done if no further changes are required.
b. For additional incoming settings scroll down and tap Advanced.
6. Update the incoming settings as needed.
a. Choose your preference for Deleted Messages.
b. Tap Account to return to the previous screen.
7. Tap SMTP to update the outgoing mail server settings.
8. Tap the Primary Server. The primary server should match the SMTP server obtained from the Customer Portal.
9. Update the outgoing mail server settings as needed.
a. Authentication should be Password.
b. Tap Done to return to the previous screen then tap Account.
10. On the Account screen tap Done to verify the account settings.