How Do I Make My C: partition Bigger By Taking Space From D:? 

Changing the layout of the hard disk make the C: partition bigger requires a few steps, and occasionally special tools.

Source: Can I make my C: partition bigger by taking space from D:? – Ask Leo!

Had this problem a while back on my work computer, the hard drive came pre-partitioned and I needed to change the layout because the main (C:) had very little storage, about 160GBs or so out of a 500GB hard drive, so since nothing was on the 500GB side just yet I just deleted it and repartitioned it, but if your case is not so simple, this tutorial should help you out.