To upgrade your subscription go to the Profile menu and click on the 'Upgrade subscription' section.
After subscription upgrading you will be refunded for the remaining days of your current subscription.
If you have canceled the auto-renewal of your subscription, you can subscribe to another one after your current subscription has expired. You can see the expiration date in Profile menu ---> Settings ---> Subscription
If you purchased your subscription via Google or Apple, you can find the subscription period in the subscription settings in your Google or Apple account.