I'd like to extend my current subscription

First of all, check whether the automatic billing is enabled. If so, you should just wait for the next payment date. Everything will be done automatically.

If you want to extend your subscription manually anyway, you can do that on the pricing page. However, note that you can extend only when there are 14 (or less) days left till the auto-renewal of your subscription. To check all the info about your plan, please visit your Account and Billing page. 

Subscriptions purchased via mobile apps cannot be extended. Only the auto-renewal option is available.