See Switching to the Outlook Mobile App if you are switching from another mobile email app to Outlook mobile app.
NOTE: If you need to remove your account from a non-recommended mail app (e.g., Mail, Gmail, Email), see Alternative Mobile Device Apps. Follow the steps for your device, but instead of choosing to Add an account, select your account and Remove it. Removing the account will also remove your account from the Calendar, Contacts, and Notes apps if they were added when the account was setup.