You can upgrade your account anytime from Trial / Free to a Standard or Ultimate account, even after your trial expires.
Step 1. Just go to Settings -> Billing -> click on Upgrade to paid plan
Step 2. Select the appropriate plan according to your use. You can compare each of the paid plans here
Step 3. Select your payment method and number of email accounts you wish to send from, and click on Proceed to pay.
Step 4. Enter your Credit Card details like Card Number, Expiry, CVC, and address and complete the payment process. Your account will get upgraded to a paid plan within few minutes.