You can't really. There can only be one C drive, and Windows wont let you change the C: drive while its active.
I would just clone onto D: and then change the positions of the drives so that thew D: drive is Primary Master, and as such can boot. It should then automatically get the C: drive letter.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.