How do open a .chm file in ubantu?

You need to install xchm, which will read chm files. (chm files are Microsoft help files.) To install xchm, open a terminal and type:

sudo apt-get install xchm  

or 

.chm Viewers
I ran into a number of .chm viewers for Linux, all available within the Ubuntu repositories.
If you are a Gnome user you may like gnochm:

sudo aptitude install gnochm

If you are a KDE user you may prefer kchmviewer:

sudo aptitude install kchmviewer

There are also some conversion tools, which I’ve had varying success with:

sudo aptitude install chm2pdf