Sir if your installation is fresh and you have not made any changes you can simply remove the installation in "magento" directory :
http://softaculous.com/docs/How_to_remove_a_installation and make a new installation by leaving the "In Directory" field blank.
>>I tried to leave the folder blank during installation but didn't work.
What error did you get ?
>> I also tried to change magento url in the magento system,but didn't work again.
This is just to edit the details in the Softaculous records and not edit the installation itself. You should first change the URL and paths in the installation and then you can edit the details in the Softaculous records here.