Not a common PROBLEM

[Iñaki]

I'm not experienced with our iMac or something but i've been with it since like 4 years ago because it's a Mac OS X 10.6.8 . So the problem occured when I tried to update the OS to El Capitano and I was surprised the update proceeded to happen. Let me get straight to the point, the problem is that when it stopped updating it showed a message that it cannot be completed or something like that. I tried to quit the installation but nope it won't happen. I need a solution for this problem, a solution that would quit the installation/update and also get me back to the normal desktop. It's an old Mac and I need serious help. It's the only device where I do my projects. I'm new to this forum and a quick help would satisfy me very well. Also, going to start up disk doesn't help. I labelled it as an uncommon problem because usually after updates it goees back to normal and even gets better but it didn't happen and I'm stuck on the loading bar cycle. Shutdown, open then it comes back never ending. Thank you! in advance.