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 don't want to wait, pay now. You can extend your subscription by a month or a year. Open the Go Pro page and click on "Extend".

Then choose the billing period.  Keep in mind, that you're going to be charged at once rather than upon expiration of your current subscription.

If you extend your subscription by the same billing cycle, nothing but for paid days is changed. However, if you extend your subscription along with switching your billing cycle, be aware of the change. Next time you're going to be charged according to the new billing period.

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