Setup Office 365 email with built-in iOS mail app:

Delete Old Email Account from your Phone (If you are migrating from another Email System):

1. Click the Settings Button on the Home screen

2. Select the Mail, Contacts, and Calendars option

3. Select your old Exchange Account

4. Press the red Delete Account button.

5. If prompted to remove contacts from your account, click “Delete from my iPhone”

Add Office365 account to iPhone:

1.  Tap Settings > Mail > Accounts > Add account > Email (for older iOS versions, Tap Settings > Mail, Contacts, and Calendars > Accounts > Add account > Email).

2. In the Add Account page, tap Exchange.

3. Enter your full email address and the description should auto-populate with the word Exchange, but you can set that to Work or whatever you'd like.  then tap Next, and then when the sign in pop up displays, tap Sign In:

4. You will be redirected to Microsoft's sign in page.  Enter your password for your email account and tap Sign In, and then Accept:

4. By default, Mail, Contacts, Calendars, Reminders, and Notes are synchronized. Tap Save to complete this setup.

5. If you're prompted to create a passcode, tap Continue and type a numeric passcode. If you're prompted and don't set up a passcode, you can't view this account on your device. Alternatively, you can also set up a passcode later in your settings.