Hi Laurent,
The best way is to remove first the driver by going to your control panel, programs/uninstall a program. Go to your AMD Catalyst Install Manager and remove it. You can download the latest driver at www.amd.com
The HD 7770 has DP 1.2 spec so technically is capable of driving up to 4x 1080p displays from the DP port. Please note that some products can differ a bit from the standard discreet graphics card and the vendor can change this for specific reasons.
Regards