If you pay for your Intruder subscription by credit or debit card, you can update the number of licenses on your account 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 'Update Number of Licenses' button:
Move the slider to the number of licenses you'd like to update your subscription to:
Any changes will be billed on a prorate basis, so you'll be charged for the difference in licenses 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.
If you're reducing the number of licenses, you'll receive a credit on your next invoice. Please note that you cannot reduce your licenses to less than the number of targets you've scanned in the last 30 days.