Your subscription is renewed automatically at the start of each billing cycle (as long as your payment method is still viable).
If you have canceled your subscription and wish to start it again, go to the Mysterium VPN dashboard by following this link and log in to your account. There, press the ‘Manage payments’ button. Press 'Update plan' on the window that appears to renew your subscription.
Alternatively, you can go to our pricing page, pick out a subscription of your choice, and proceed with the purchase there.