At any point of time if you wish to change your existing plan all you need to do is tap on the hamburger icon and then on Business settings. In Business settings you will find Plan. Tap on it and you will get to see your existing plan details.
If you are on a Free plan, you need to tap on "Change plan", then select the plan you want to switch to and then proceed.
However if you are already on a paid plan and you want to downgrade, you need to first cancel your current subscription and then follow the above mentioned steps.