How do I update LTspice IV for macOS?

LTSpice for macOS has finally been updated to LTspice XVII as of October 25th, 2018. I have verified that it has resolved the model update bug, as well as most other noticeable issues that had cropped up in the last release.

So the updated answer to this question is to simply download the latest version - it fixes the model update crash and many other issues as well. Even the icons have been tweaked.

The new URL (since Linear Technology has since become part of Analog Devices) to download the latest LTSpice version (I would be surprised if the self-update function worked, given the change) is here.

I also was stuck on this and decided to trace it down. So what the Software Update does is to download a file named LTspiceAppUpdater which is put in /Applications/LTspice.app/Contents/. This waits for LTspice to terminate itself. It then simply downloads the most recent LTspice.dmg from Linear.com in the same location and extracts the files. So basically it replaced your recent installation with the same copy. Then on restarting it just hangs in an infinite loop. I could not figure out where exactly the version is checked, but since I deleted each and every trace of LTspice and it still croaks it's most likely that they just look into some token in the program code itself.

tl;dr It's a bug in the implementation and you can only get rid of it by reporting it to Linear.com. Though I think it's unlikely they will fix it in the near future - if ever. Just click away the message at start and have a look for a new version next year.

There's a new version available, but that crashes permanently (not only with my installation). Here's a link to the old version: https://www.heise.de/download/product/lt-spice-iv-65702/download (it's a German magazine and it should be safe to download in any case) Or even better: use the link in @HiromichiMatsushima's answer. This seems to be a much more stable release.

The build date/time is hard coded in the executable. Open LTSpice executable in a hex editor and goto offset 0x49FA4A and change 0x36 to 0x38. Now it will think the build time is October 2018 and it won't popup that annoying message anymore.

LTspice MD5 = e7a2de8bb82e2419e981ed425c359626