When your trial expires you can upgrade your account to PRO plan by going to 'Subscription' and entering payment information:

Please enter your payment and billing information and select a payment plan. You can choose to pay annually or monthly. The cost of My Hours subscription is $6 per user per month when billed annually and $7 per user per month when billed monthly.

By clicking 'Activate Pro plan' you will upgrade your account to PRO subscription. Your credit card will be charged the displayed amount.