yo go into disk management and right click on the drive... it should give you an option to change the drive letter. im not 100% sure you can do this with your system hard drive though, example:
youve installed windows on c:, therefore you cant change the letter
youve got an extra drive... say d:... you can change it to say Z: but this could cause minor problems, like where a media player looks for music if you have the music on the extra hard drive... simple change, just change the drive letter on where it looks for the music.