If you're on a trial, you will see on your dashboard a button that says "Upgrade Plan" on the upper right corner, which will take you to the right page for this.
If you're already subscribed to a paid plan and wish to upgrade, you can click 'Settings' in the top bar and go to Billing → Pricing Plan.
On the Pricing Plan page, you will find all the information regarding our current plans and some FAQs at the bottom of the page. You'll be able to upgrade to your desired plan by clicking on 'Upgrade'.