I would like to modify a Joomla extension for my own company website but i don't understand something. The extension is under GPLv2 or later License witch i thought meant you are able to change it if you would like but when you open up the file is says it not to be modified isn't that the point of the GPLv2 or later License? Also some files have copy right to such and such person am I able to change those extensions I'm not going to distribute them its just for my company website so how does that work? Also one Joomla's website in says that some are Non-Commercial what does this mean when its a GPLv2? 
Sorry for so many questions I just don't understand how this works out in the really world.