If you have paid via Steam or purchased a one time code:
If this case you will need to open a support ticket here and a staff member will try to give you the equivalent value of your remaining time.
If you have paid via Xsolla please follow these steps:
First head to the manage account section here and select to manage your subscription:
Select to manage your subscription:
Select your ESEA subscription:
Select to change your plan:
Now you can select the new plan that you wish to purchase, any remaining time will be credited toward the new subscription:
Please select the new package that you wish to check out with and follow the steps on screen.