To upgrade your Iconfinder Pro membership, please follow these steps.
Log into your account and go to Your account page, by clicking on your avatar and choosing "Your account" at the top right corner of the page.
Click the Change Plan button from the right side of the page in order to change your subscription plan.
Choose which Pro subscription you want to continue with. On the right side of the screen, you will see the details of the new subscription plan and the prices. When you finish selecting the new subscription plan, click on the "Upgrade now" button to finish the process. In this case, you want to upgrade from Pro Starter to Pro Unlimited.
To do the opposite process (downgrade), please see How to downgrade your subscription.