If you're accessing your account on a mobile device, open the menu via the 3 lines in the top right corner of the screen:
The Account login link is in the bottom right corner of that menu:
From here you can enter your email address and you'll be emailed a unique onetime code to sign into your account:
Once logged in you'll see your next subscription order details at the top of your Account page, click the View and manage order button to go to the subscription management portal:
From there you'll be able to manage your subscription with all the same tools as the desktop website (lots more how-to's on the other Help Centre articles here if you're unsure about anything!).