Do you have an active Standard Subscription and you want to get Pro?
If you want to upgrade your current one:
Go to 'Profile' tab - subscription tab would be right below your avatar place,
Select Subscription of the higher tier,
Complete the purchase with your paywall
You will receive a corresponding notification if the purchase is successfull
You are Pro now!
The billing amount will be changed and the upgrade to be applied right after you complete the purchase. In the end of the current billing period the corrected amount will be taken from your bank account.