If you purchased a Premium membership and realise that you have purchased the wrong run time: don't worry, you can still change it. It is technically not possible to change an already running subscription due to limitations from the payment platform side but there is the following solution:
1) First step to change your Premium subscription model is to cancel the current one
2) The second step is to ask for a refund
3) After completing these two steps, you can buy a new subscription.