Step 1: Open your profile my signing in and selecting "Account Information". Click the green link "Change my Subscription" on the right-handside.
Step 2: A popup will come up as per the screenshot below, choose the orange button "change my plan" if you are on a free trial or expired account.
If, however, you are on the Premium plan and wish to upgrade to the Pro plan, you need to cancel your subscription first, or you'll be billed for 2 plans at the same time.
Step 3: Select the membership you would like to buy. Note that if you want to pay yearly please select the "Pro: $490 / year" link as per the circle below...
Step 3: A checkout page will open, as per the screenshot below. Input your coupon code in the space provided and click "Update" next to it to claim your discount.