After help from many & learning some thing, I finally able to upgrade from catalyst 11.5 to catalyst 11.6 in Ubuntu 11.04.
Thanks to those who helped me.
You need to first have to uninstall the ati drivers comes with Ubuntu 11.04.
Go to Synaptic Package Manager & search field type fglrx.
It will give some results & you will see 4 files with names starting with flgrx.
Out of the 4 , 3 is already installed & 1 is not installed.
fglrx, fglrx-amdcccle & fglrx-dev is already installed
fglrx-modaliases is not installed. (You can't install fglrx-modaliases with the other 3)
Ok, now go to AMD site & in support & drivers,
http://support.amd.com/us/gpudownload/Pages/index.aspxselect desktop graphics-> Radeon HD series -> Radeon HD 6xxx Series PCIe -> Linux x86_64 & click display results.
It will open a page with AMD Catalyst 11.6 Proprietary Linux x86 Display Driver, file size is ~72MB, version 11.6, direct link is below.
http://www2.ati.com/drivers/linux/ati-driver-installer-11-6-x86.x86_64.runDownload the file & keep it in desktop.
To install this file, first have to make it executable & then run in terminal with root access.
chmod +x ati-driver-installer-11-6-x86.x86_64.run
sudo sh ati-driver-installer-11-6-x86.x86_64.run
It will run & will tell you that already drivers are installed & ask you to uninstall it or go for force install.
DON'T GO FOR FORCE INSTALL.
Now go to Synaptic Package Manger & select the 3 fglrx, fglrx-amdcccle, fglrx-dev which will have a GREEN box in front of it, check the 3 files & select Mark for complete removal. Sometimes it will ask you to remove other also if you select one file of the above.
fglrx-modaliases will have a GREY box, which means its not installed.
Click Apply & it will uninstall all the above 3.
Then again run
sudo sh ati-driver-installer-11-6-x86.x86_64.run
Now you will be able to install 11.6.
After installation is complete run
sudo aticonfig --initial -f --adapter=all
sudo reboot
After system reboot, you can able to login without any problem.
To over clock you must enable ati for OC/UC.
sudo aticonfig --od-setclocks=900,300 --adapter=0
Just run the above code in terminal to under clock memory to 300 Mhz & over clock core to 900 Mhz.
It will tell you that you have to enable OC/UC.
I am not sure here about the correct code as it is displayed in the terminal window itself, just use sudo with it for root access & to enable OC/UC
sudo aticonfig -od-enable
Now you can OC or UC as you want.
Also if you want to see temperature of all GPU's, use this code.
watch -n 5 aticonfig --odgt --adapter=all
Instead of 5 use 1 for 1 sec & it will update all the core temperatures every second.
You run it in separate terminal, so you can check it easily.
I hope this will help those who also searching & trying to install 11.6 in Ubuntu 11.04.
Screenshot showing temperature window & 8 cards mining in Ubuntu 11.04 using 11.6 catalyst driver.