I'd like to upgrade my annual subscription
You can upgrade your annual 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.
However, if you subscribed through the Android mobile app, you can downgrade your subscription at any time. All remaining days will be automatically converted to additional days of the new plan at an equivalent cost.
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.
Here you can check how to upgrade your plan in mobile apps.