First, make sure you have a Pro account plan.
From your dashboard, go to the Left-hand side and select Plan and Billing.
Then under Usage, you'll see all available user seats. Next to each user seat, you'll see the option to either Remove seats or + Add seats.
To add seats, click + Add seats. You'll see a pop-up where you can choose how many seats you want to add to your account. After you’ve selected the amount, click on Add seat(s).
To remove seats, click Remove seats. You'll get a pop-up where you can choose how many seats you want to remove from your account. After you’ve chosen the amount, click on Remove seat(s).
You will be charged monthly for each seat you purchased until you remove them from your account. Each new billing cycle starts on the 15th of the month, so if you'd like to remove a seat for the following month, you have to do so before the 15th.