I'd like to upgrade my annual subscription

You can upgrade your annual subscription at any time with no extra 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 remainder of your plan to switch to a better version. Your subscription's duration will be reduced, and your next payment date will be changed. Be aware of it.

The conversion of the remaining days of your current plan is done according to this ratio:

Essential → Plus: 2 days of Essential equals 1 day of Plus
Plus → Premium: 2 days of Plus equals 1 day of Premium
Essential → Premium: 4 days of Essential equals 1 day of Premium
Premium → Ultimate: 4 days of Premium equals 1 day of Ultimate

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 of how the upgrade looks 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.

Please note that if your subscription has 4 or fewer days left until the next renewal, you will not be able to upgrade without extra charges; you'll be offered to purchase a paid plan instead.

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