How to install Radeon Open Source Driver?

Removing the watermark is very simple with a bit of cut and paste, then a command:

How to remove the “AMD Testing use only” watermark?

If you did want to use the open source drivers (which are not as feature complete) you just need to remove the fglrx driver:

sudo apt-get remove fglrx*

then ensure the open source one is installed. It should then be picked up:

sudo apt-get install xserver-xorg-video-ati

The problem with the HD 7000 series is that support for it in the open source driver only arrived around Christmas, so unless you are running the xorg-edgers PPA you would be better off with just removing the watermark for now.

to remove the driver not supporting your grafics card: read the part and follow "Problem: Need to purge -fglrx" of this site: https://wiki.ubuntu.com/X/Troubleshooting/VideoDriverDetection

ensure with synaptic if "radeon" driver (open source) is installed the open source driver radeon should work then properly i hope :D