If you have subscribed to the Professional plan, but wish to upgrade to Professional+ to test out the middles - here is the way:
Navigate to your user dropdown menu:
Payment -> Plans and press on Choose for Professional+. The amount that you have already paid will be pro-rated accordingly. Just make sure that you have at least 5 days remaining on your current subscription. If there are less days, wait for the current subscription to end.
However, note that the backward change will not be that simple. If you decide to downgrade to Professional in the future, please contact us.