Assuming Windows 7 here...
Needs to be done in two steps, first you have to Shrink D: in order to free up some space for C: to Expand on.
Computer (right-click on icon)> Manage > Disk Management > Select D: > (if available) Shrink
Volume (set size).
AFTER THAT do the same, select C: > Expand
However, BEFORE doing this, I would recommend that you scrutinize you usage of C: and see if there is anything you could reallocate to D: ...
also, tutorials. Partition or Volume - Shrink Partition or Volume - Extend