Once a file has been downloaded, it should normally install in a reasonable time. Typically around 45 minutes, more or less, is required for an OS installation. The first thing I would do is to put the installation file into the Trash and empty it. Then try another download. What often occurs is that these very large files can get corrupted during the download process, especially if Wi-Fi is used. It is just too slow. So, try doing the download with a wired Ethernet connection. If that also fails, then repost and other things can be considered.