You can do it from the Payment Method screen available under the app's Subscription menu. 

From your dashboard, click on your team name (dropdown) at the top left menu. 

Select "Subscription" from the menu.

On the subscription screen select the plan you want to subscribe to or upgrade to:

A the bottom of your billing information you will see a field to enter "Coupon". 

Enter your coupon code and click on "Subscribe"

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