Mac OS new update stuck in downloading

Stop the download and boot into Safe Mode according to How to use safe mode on your Mac and try downloading and installing from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

I tried the update this morning and it hung up about 3/4 through the download. I quit and did a few errands, then came back and tried again. I seemed to get hung up in the same place, and just as I was about to stop, it finished the download and started the install.