To add more time, check out again using the same email address you used before and select Extend Subscription at checkout. The new time is added on top of your existing line, nothing is reset, and you keep the same login.
If you want more connections so more devices can watch at the same time, or you want to change your plan length, the easiest path is to contact us so we set it up correctly on your existing line.
Open a support ticket using the Contact support button with your order number and tell us the change you want, and we will take care of it for you.