Is there more unallocated space on the drive? Then goto disk management, click on the c partition and extend it. If there is no more space on the drive, you need to buy a bigger one and clone the old one to the new, for example with acronis.
not too much help you can get from build-in disk management, you can extend the C only when you have unallocated space behind it. So 3rd party software is the better choice, but backup your data before you partition is always recommended. Here I recommend AOMEI Partition Assistant which works fine for me, you can find the software from PCworld: http://www.pcworld.com/article/257248/aomei_partition_a... they speak highly of it.