To upgrade your Gremln plan, you will do this via your Account Settings page. Hover over your username in the navigation bar, and select Account Settings.
Once there, under Account Info you can view your current subscription type and have the option to add extra team members. If you're looking to upgrade your account, click the plus sign to add the number of seats to match the type of subscription you're looking to upgrade to. Then click the purple Update button.
You will then be taken to your cart, where you can view and edit your purchase. You can also select on what schedule you'd like to make your payment. Then click Continue to Checkout.
You will then be redirected to PayPal.com, where you will have to have a PayPal account and login here. Your subscription will then be active and Gremln will automatically draft from your account on the recurring basis you chose starting from the day you sign up.