Unpartioning a hard drive.

I have an older computer and the hard drive is partioned.   The smaller part of the partion is where all of the OS software and updates are located (c:).  I still have a lot of room on the other part of the hard drive (d:).  I was wanting to either allocate more memory to the c: drive or remove the partion all together, and I don't want to reformat my hard drive.