This procedure is applicable for iOS devices (iPhone, iPad, etc.) running iOS 11 and higher. The screenshots shown below were taken from an iPhone 7 running iOS 11.0.1. Other devices will be similar.
If you have iOS 11 and experience issues with email connectivity please update to the latest version of iOS. Apple has acknowledged a bug with the Mail app and email connectivity effecting Exchange in the following article: https://support.apple.com/en-us/HT208136
Prior to starting this procedure, obtain your account-specific information from the Customer Portal according to the following article:
How to Obtain Exchange Account Information
To edit Exchange email account server settings on an iPhone or iPad take the following steps:
1. From the Home screen tap Settings.
2. Scroll down and tap Accounts & Passwords.
3. Tap the Exchange or other Email account to update.
4. Tap the email address next to Account.
5. Update the account settings as needed and tap Done.