You should be using source control at all times. This lets you trigger the update, close Unity and then go study the source code differences, then instantly revert.
The only place that isn’t gonna be obvious is if you have DLLs that get updated, and in that case you can unpack the DLL and compare before and after. And if it’s a DLL, it’s unclear how you could do anything but update it.
Using source control properly is also the ONLY way you should ever update Unity to a newer version, otherwise you may destroy your project permanently.
Please consider using proper industrial-grade enterprise-qualified source control in order to guard and protect your hard-earned work.
Personally I use git (completely outside of Unity) because it is free and there are tons of tutorials out there to help you set it up as well as free places to host your repo (BitBucket, Github, Gitlab, etc.).
You can also push git repositories to other drives: thumb drives, USB drives, network drives, etc., effectively putting a complete copy of the repository there.
As far as configuring Unity to play nice with git, keep this in mind:
Here’s how I use git in one of my games, Jetpack Kurt:
Using fine-grained source control as you work to refine your engineering:
Share/Sharing source code between projects:
Setting up an appropriate .gitignore file for Unity3D:
Generally setting Unity up (includes above .gitignore concepts):
It is only simple economics that you must expend as much effort into backing it up as you feel the work is worth in the first place. Digital storage is so unbelievably cheap today that you can buy gigabytes of flash drive storage for about the price of a cup of coffee. It’s simply ridiculous not to back up.
“Use source control or you will be really sad sooner or later.” - StarManta on the Unity3D forum boards