Welcome to the forums Debosheer
You should not login as root, in Ubuntu you use sudo to gain root access rights. To run a command as root you type
. You also need to make sure the file has executable attributes ... the chmod command will allow you top change the attributes. Type
for more information on the chmod and sudo commands.
If you are changing graphics drivers you will normally need to do this by pressing ctrl+alt+f1 and login as your normal user. The GUI will need to be stopped but instructions for this are normally on the website you download the driver from.
You may be able to install the restricted drivers from the GUI ... I think its in system administration. If it is available then this is the easiest method to use.