Use the computer management console to change the drive letter.
There's a Microsoft article on the subject at this link:
Basically you open up your Control Panel, double click on Administrative Tools then double click on Computer Management.
In the left hand panel of the Computer Management console, click once on Disk Management under the sub-heading of Storage.
You'll see your drives listed in the right hand panel.
Right click on the drive and choose the "Change drive letter and paths" option. You can alter the drive letter in there.