By going here

https://github.com/TriumphLLC/Blend4Web

how do I download just what has changed since the previous version?
Working with github need some preparations and additional routine but when you get familiar with it updating b4w sdk will be as easy as pie

These tutorial are exactly what you need:
https://help.github.com/articles/set-up-git/
https://help.github.com/articles/cloning-a-repository/
After these steps the only thing you should do for getting updates is to use "git pull" command from your local directory with sdk.
There are several github desktop clients for different platforms like https://desktop.github.com/ which may save you from using terminal.
We'll answer any questions if tutorials above are too confusing