How to add more disk space to the local drive?

I am using Windows 8 Single Language 64 bit. I have > 350 GB Unallocated space in my hard disk. I want to add this > 350 GB unallocated space to C: drive where Windows is installed. I have checked using 'Extend Volume...' in 'Disk Management', which is currently disabled. Is there some simple way to do this? Kindly don't answer like backup your data and format your hard disk/drive, use third party software, etc. Kindly help me and ask for any further clarifications if needed.

In this case, I transferred my files back to C: drive, deleted E: drive, extended C: drive and created E: drive again (with the space remaining). This is because a partition can only be extended if there is space next to it (contiguous).