depends on what file manager you use and whether you are rooted or not..
you need to be on a rooted ROM to view the contents of the /system folder where the build.prop is located
second is you need a root file explorer like es file explorer, most roms come with one, each file explorer gets to the root differently, if you are on my rom (n00b) for example, I have miui file explorer, to get to /system you need to click the drop down at the top of the screen in file explorer and choose internal, then you can navigate to the system folder.
ES file explorer, you need to go to settings in that app and enable root browsing, then you can use the up button to get to the internal side and the system folder.