How do I delete an email address from Facebook App login page on Android?
First log out from your account and then follow the following steps as given on:
[https://m.facebook.com/help/670304209712363?helpref=uf_permalink
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To remove an account from your phone or mobile device:


Go to the login screen on the Facebook app 
Tap [Account to be removed]
Tap Remove account from device


For making for account as default just first logout and then login into your account.