Licenses can be upgraded to higher tiers through our store, by clicking on the "Upgrade" button:

A new modal opens up where you can choose available upgrade plans.
Please note that the Lifetime purchases can't be switched to Subscription plans, so if you have a Pro Lifetime plan, you cannot upgrade to Developer Subscription, only Developer Lifetime. Also, depending on the license you currently have, different options will be offered in the modal.
What you should know about the upgrade?
The price that you will pay for the upgrade will be a pro-rated charge, meaning you won't be paying for the price difference between licenses, since we cannot refund the time spent on the license you currently have.
Example:
You bought a Subscription license which costs $69 (roughly $0.19 per day).
After 100 days, you want to upgrade to a higher-tier license, which costs $199 (roughly $0.55 per day).
Time remaining on the old license (what going to reduce the upgrade price): $50.35 (265 * 0.19)
Time spent on the old license: $55 (100 * 0.55)
Time remaining on the new license: $145.75 (265 * 0.55)
So, the price is not new license - old license (199-69=130), but instead a pro-rated charge, and you are going to be charged around $147.75 - $50.35 = $97.4.
If you're upgrading from a yearly subscription license to a yearly subscription license, it still renews on the same date (1 year after the initial purchase) and your purchase code remains the same. This is basically why the prices are pro-rated.