The modification are taken in effect at the start of your next billing cycle, and the new amount will be billed to you. You will not lose your rollovered that, also if you change to a no data plan, or if you change the coverage.
Go to your account → My Plans → Click on Manage on the plan that you wish to change → Adjust your plan
Go to your account → My Plans → Click on Manage on the plan that you wish to change → Adjust your plan and you will have the opportunity to cancel or edit your changes, follow the instructions
For a mobile, if you never had the data you must configure the APN
For home internet, restart your modem, unplug the power cable of your Fizz's modem for 1 minutes, when plugged in back wait 3-4 minutes giving the time to the modem to be full operational before try ing to connect to internet.