Q: Update freezes before completing download. Please advise.

Hi, gillfromtowcester!

Thanks for using the Apple Support Communities. From your post, I am understanding that you are attempting to update your OS X to the latest El Capitan release, 10.11.5, but that it doesn't seem to complete. I rely on my Mac every day, and know staying up to date is important. I'm happy to offer some options that may help!

First, try updating the OS X software through safe mode. Instructions for getting into safe mode can be found in the article linked below. Even though the title isn't directly related to your situation, it's still a good step to take; safe mode runs a check of your system, and will disable certain software that could be causing a conflict.

Try safe mode if your Mac doesn't finish starting up - Apple Support

If you're still not able to update while in safe mode, I'd suggest setting up a brand new user account and trying the update there. The following article will give some information on why setting up a new user account can be helpful, as well as providing instructions on how to create one if you need help.

How to test an issue in another user account on your Mac

Have an excellent day!