There are two different scenarios for this question:
1. If you purchased a Premium membership and realize that you have purchased the wrong run time: don't worry, there is a solution. It is technically not possible to change an already running subscription due to limitations from the payment platform's 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 the new subscription
2. If you decide during the run time that you would like to change to another model but cannot receive a refund due to the 14 days cooling period being over: please cancel the subscription so it will not renew, let it run out, and then purchase the new model.