@MaruRuiz
You say you are Premium user, but there should be the Premium badge shown next to your username here.
Do you mean you are premium on the web version ?

@T-S-4 You should be able to tap on the icon of heart or your profile picture, if you are logged in and there you can delete all your favorites by swiping them to the left and tapping on delete button.

Please provide any screenshot if you mean anything else.