There are Extensions that allow you to do this, but due to the restrictions that it has imposed Google (the very motherfuckers) a Cuba I can't get them so I have to do this by hand.
For this we edit the file /usr/share/applications/chromium.desktop and we look for the line that says:
And we put it like this:
Exec=/usr/bin/chromium --user-agent="Mozilla/5.0 (X11; Linux i686) AppleWebKit/535.1 (KHTML, like Gecko) Debian Chromium/14.0.835.202 Safari/535.1" %U
If we open a tab and put:
We can see the User Agent which more or less in my case should be like this:
--user-agent="Mozilla/5.0 (X11; Linux i686) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/14.0.835.202 Safari/535.1"
If you compare it with the line that I left above you will see that I added Debian and I substituted Chrome by Chromium.