You can easily upgrade your account by changing the number of dashboards in Account Settings under the Subscription tab.
You can add more dashboards at any time and they will be added to your bill in the current billing period. At the bottom, you can see what your New monthly price will be.
NOTE: The price of these dashboards is based on the days spent using them, and proportional to the time left until the end of the billing period, i.e. if you’re only going to use a dashboard for half a month, you’ll only pay half the price; if you activate one 3 days before the end of the billing period, you’ll pay one-tenth of the regular price, etc.