There is no ETF if cancelled within the first thirty days of service. If cancelled after that, but before your two year contract is up, it's $400.
Wanting to cancel after one day seems a little odd. Are you having major problems with the service or something? If you are, there are people on here who can help get things figured out.
Running out of data is not a Windows problem, it's a user problem of not remaining cognizant usage, using the internet willy nilly, and not having things set up right. The blame lies with the user, not the equipment.
I'm 18 days into the month and have used 4.3GB of Anytime and 7GB of Bonus Bytes, and most of both of those was used with upgrading my folks' Windows 7 computer to Windows 8, then doing all of the updates, then upgrading it to 8.1, again doing updates afterward. Then I updated a nearly ten year old Dell to W7 from Vista, then updated, then upgraded it to Windows 10, then updated. The latter computer was just to see how well a nearly ten year old computer would run Windows 10, but also so I could give it away as I don't use it and probably never will again. There might be a local family with a kid that could use a computer at home for school work, but can't afford one.
Probably at least 80% of my usage this month was all of the upgrades and updates for those two computers, so my normal usage would be the rest.
A Windows problem? No, a user problem.