If you already purchased a premium account, but want to purchase additional licenses (or switch from monthly to yearly), all you have to do is go back to the checkout screen to update your subscription.
To do this, log into your account and click on the name of your plan in the top right corner of the web page (it will say either "Pro Plan" or "Business Plan"). Then, click the link to manage your subscription.
Next, you'll be taken to a page very similar to our Pricing Page, which will show you your current subscription set-up. Click on the link at the top to update your subscription, or click on the Pro or Business pricing tile. Either option will take you to the checkout page. You can also switch from monthly or yearly to see the difference in price beforehand.
Once you are on the checkout page, select the type of additional licenses you wish to purchase - monthly or yearly - and then add the appropriate number of licenses to your current subscription. Be sure that everything is to your liking before clicking the "Subscribe" button. Once you do click that button, your subscription will be updated based on your checkout selections.