If you reach your bandwidth limit in any given month, you will receive a page that says "Bandwidth Limit Exceeded". You will not be charged any extra fees for reaching your bandwidth limit.
To remove the bandwidth limit notification page, you can either wait until the next months bandwidth reset or you can upgrade your hosting plan to the next available hosting plan.
Upgrading your hosting plan can be done from the client area by going to "My Services" and selecting the hosting plan and from the Details page you should find upgrade options.
If you choose to upgrade your hosting plan, it will instantly be upgraded once payment has been received.