How to upgrade hosting plan?

[kami@]

Hi,
I am new in plesk and I like to know if there is a way to upgrade customers hosting plan.
Lets say I offer 2 hosting plan of 100mb and 200mb and a customer wish to upgrade his hosting plan from 100mb to 200mb. How do I do that?????

I have been loking all over in plesk cp for window and do not seem to be able to find a way to do that.

Thanks