Instructions
Turn your PS3 on, and log on to the profile with the name you wish to change.
Select the "Settings" option at the far left of the PS3 main menu, and find the name of the profile you want to change yet again.
Press the "Triangle" button on your PS3 controller when your target profile is highlighted, and select the "Information" option on the drop-down menu.
Select your profile name on the information screen. You will now be able to change the profile name to whatever you like.