I am sorry to hear about having this problem, but I think I can help you. First, a little bit of general information. Two types of system downloads are offered for installation. The first is a system update which fixes and updates the current operating system. The second is a system upgrade which replaces the current OS with a newer OS. It sounds like that is what you are attempting to do.
Updates and upgrades should normally take no longer than an hour or so to download, plus maybe another 45 minutes to install. What often happens is that a person attempts to do their download via a wireless (Wi Fi) connection, which is what it appears that you have.. This is simply too much data for wireless. What results is that the data is either incomplete or is corrupted and won't load. I recommend that you move the download corrupted file to the trash and do another download with a wired USB connection to the modem/router. If necessary, you may have to temporarily move the computer. If this is not possible, then be prepared to have to do multiple downloads via Wi Fi until you get a clean one that works. (Be sure to put each one that doesn't't work into the trash and then empty it.) I hope this is helpful. Good luck.
