On desktop:
In the top right, locate and click the "Premium login" button.
Enter you username and password. Make sure you use the same e-mail address and password that you used to sign up for Premium.
Here's a quick video:
On mobile:
Locate and tap the "hamburger" menu in the top right.
Tap the "Already a Premium member?" link
Wrong e-mail address: If you have several e-mail addresses and are not sure which one you used, try to find the confirmation e-mail from us with the sign-up payment details.
Wrong password: If you have forgotten your password, try to reset it on the sign-in page.
If neither of these suggestions helps, contact our support for further assistance.