How do I edit my partition?

What you want to do is SHRINK the D: partition size, then EXTEND the C: partition size.

Both tasks can be performed using windows native Disk Management tool, but the problem is that the Windows tool will create the Unallocated Space from the D: partition at the end of the partition and you will not be able to Extend the C: partition into that Unallocated Space because the remaining D: partition is "in the way" as far as the Windows tool is concerned.

You can use a third party tool like Minitool's Partition Wizard from the bootable CD to do the job and overcome Window's limitation.
Instructions here: How to expand a partition?
Bootable CD here: Partition Wizard Bootable CD allows user to manage partition directly with partition manager bootable CD.

There are other programs that will do the same thing, but this is the program that a lot of us here use.

Make an image of all disk partitions to an external HDD first just in case anything goes wrong.

kado897 said:

Make an image of all disk partitions to an external HDD first just in case anything goes wrong.

Aint that the truth, it's a must do before editing partitions, so many things can go wrong, especially if you're inexperienced. Read all the instructions carefully.

Based on what was said you can also proceed as follows:

1. Shrink the D partition with Disk Management

2. Image the shrunk D partition. Easiest is to use Free Macrium

3. Delete the D partition so that you get unallocated space

4. Extend the C partition by the amount that D was shrunk - but not more, rather use a little less. Reason: If you use more than the shrunk amount, the image will not fit in step 6.

5. Create a new D partition from the remainder of the unallocated space - this can be done with Disk Management.

6. Restore the image of D into this new D partition.

I would prefer this solution because you only need Disk Management to manipulate the partitions - and Disk Management is safe. Partition Wizard is a nice program which I use a lot, but for the uninitiated it is easy to make a mistake and to muck up all partitions on that disk (has happened to me in the beginning too).