How often is the Unity software/code style updated?

How often is the Unity software/code style updated?

I’m interested to know how often Unity is updated, and how much is usually updated at a time.
I’m going through a slightly out of date Unity gaming book, and some of the buttons and type of code mentioned are different. How often do you have to brush up on Unity when it updates?

Thanks!

If you go to Download Archive you’ll see a list of all the different versions, their release date, and the notes explaining what features were changed from the previous version.