All users have at least 2 free email addresses: one that ends in @protonmail.com and another one that ends in @pm.me. You can’t disable your original free addresses.
If you joined ProtonMail prior to 2016, you also have an @protonmail.ch address.
To manage your original free email address, go to Settings -> Account. Here you can edit your address display name and signature .
Users with free accounts can only receive emails at their short domain @pm.me address, whereas those with paid accounts can also send messages from their @pm.me address. To manage this address, go to Settings -> pm.me.
To upgrade your free account and benefit from premium features, go to Account -> Dashboard and select a new plan.