Through disk management
Go to Control Panel/Adminstrative Tools/Computer Management and click on Disk Management. Scroll to the disk you want to change and do a right click. From here you may change the drive letter. You must change it to one that is not in use. If you want to change to one that is in use, go to that one first and change it to an unused letter and move the other drive to that position. You may do this with all but the boot drive.