Question: how to change your default email app on iphone?
Answer: Step #1. Launch Settings app on your iPhone/iPad → Scroll down and tap on Mail, Contacts, Calendars.
Step #2. Again scroll down and tap on Default Account from under the MAIL section.
Step #3. Select any account you want to set as Default Account.
Step #4. ... 
Step #5. ... 
Step #6. ... 
Step #7.