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 so on the pricing page. However, please note that you can only extend your subscription when there are 30 days or fewer remaining before the auto-renewal of an annual plan, or 14 days or fewer remaining before the auto-renewal of a monthly plan. To check information about your plan, please visit your Settings and billing → Subscriptions page.

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