Change "device" and "OS" in Google Analytics?

I'm trying to change some variables using javascript in a web. For example, is easy changeing referer, system height or weight using javascript. But when I change userAgent, wich seems to be the variable that uses Google Analytics to analyze if you are using mobile or PC to navegate on a web, or your OS, this doesn't seem to work. Google Analytics still detects the device I really use and not the userAgent that I change in my web using JavaScript.
How can I change this? I want Google Analytics to think that my visits come from mobile even if they use PC, for example. Can I do this? Change the referer and Display size seem easy, but not with device an OS.