If you don't remember your username or password, you can click on the Forgot password or can’t sign in button in the sign-in with Email dialog.
You should enter an email used during the sign up or a verified phone number. You can also enter your username in the search field if you remember it, but can't remember other details.
You will then see several ways of receiving a password recovery link.
Registered email address
A phone number linked to your account
If you signed up through a social login, you should be able to log in using the same credentials