If you mean that you have cash credit on your account and want to use it to buy a membership plan then just make sure that the cash credit balance is at least as much as the cost of the membership and then at checkout you will be presented with the option to pay using your cash credit.
That option will not appear if your cash credit balance isn't at least equal to the checkout cost.
See here for example:
If you need to contact 48 support about this then see the links in my signature below.
