During your initial account setup, you choose to enter your email or phone number as your login.
If at some point you need to change your login credentials, you can manage this very easily.
Navigate to your user profile on the bottom panel.
Tap the settings button on the top-right.
Select settings.
Within your settings, select Account access and security.
In the Account access and security window, select Account access.
You will see Phone number and Email here.
Tap either and you will see an option to change them.