I'd like to upgrade my annual subscription

You can upgrade your annual non-professional or professional subscription at any time with no extra money payments. All remaining days of your current plan will be automatically converted into equivalent value days on the new tier. You don't lose anything, just use the remaining amount to switch to a better version. Thus, the remaining period of your subscription will be reduced and your next payment date will be switched. Be aware of it.

Remember, it's not a free trial. Once upgraded, there will be no option to revert this action. However, you'll be able to set a downgraded plan for the next billing period.

Here is an example how the upgrade looks like on the website. To check how it works in your case, click the Upgrade button on the plan you'd like to switch to. Don't worry, nothing will happen without your confirmation.

If you want to switch from a non-professional subscription to a professional one, you should purchase a new subscription. It will be activated immediately. All unused days of your previous plan will be converted into days on the new tier and added to the new subscription, extending it. The next billing date will be adjusted accordingly. 

Here you can check how to upgrade your plan in mobile apps.