http://answers.microsoft.com/en-us/windows/forum/windows_7-performance/how-can-i-edit-the-os-name-in-the-boot-manager-of/19891f91-0076-e011-8dfc-68b599b31bf5

a.        Start the Operating System (ex: Windows 7) that you want to change the name of in the Windows Boot Manager screen.

b.       Click Start and type cmd in the start search box. From the programs list right click on cmd and select Run as Administrator.

c.        In the command prompt, type bcdedit and press enter.
NOTE: This will let you see what the name of the {current} operating system name is. This will be the operating system that you are currently in (ex: Windows 7).

d.       Now type the following command and press enter.
bcdedit /set {current} Description "name you want"

e.        In the elevated command prompt, type bcdedit and press enter.
NOTE: This will let you see and verify the new name of the {current} operating system.