You can upgrade or downgrade subscription on your Profile page.
If you wish to upgrade your plan, you'll be charged for it automatically (prorated payment). You will be charged the difference between what you've already paid and the cost of the new plan for the remaining time in your current billing cycle.
Example: Upgrading from Small to Medium Plan. If you already used 2 credits from Small Subscription, after an upgrade, you'll have 13 credits left (15 credits - 2 credits = 13).
If you wish to downgrade your subscription, all the unused credits will remain on your balance.
Example: Downgrading from Large to Small Plan. If you have 20 credits left from a Large plan, they'll stay on your balance after you make a downgrade.