If you pay for your Intruder subscription by credit or debit card, you can upgrade your account from the Essential to the Pro plan at any time on the billing page inside the portal.
Click to open the dropdown menu in the top right corner and select 'Billing':
Click the 'Upgrade to Pro plan' button:
Any changes will be billed on a prorate basis, so you'll be charged for the difference in plan cost for the current month (as shown in 'Amount you'll be charged today') immediately. It will also show how much your next monthly payment will be.