If at any time you need to change the resources you've got contracted on your Coudwebs, just access your Client Area and modify them.
If you increase the number of websites and/or mailboxes, you'll receive a one-off invoice for these new resources for the days remaining until the monthly billing of your Cloudwebs. If you keep these resources, you will be sent a consolidated invoice for everything that you have contracted at that time.
For web space, since your consumption is measured at the beginning of the month, you will be sent a full invoice that relates to the amount for that extra space for that entire month.