I have an area of fat on my back at the nape of my neck. How. Can I get rid of it?

Liposuction. Liposuction is likely the best option. However, not treating a specific cause of fatty deposition in this area, if there is one, likely will result in a suboptimal correction after liposuction.
Buffalo hump Reduced. You may be referring to the so-called "buffalo hump" on occasion this can be treated with liposuction. Non-surgical methods are evolving and may offer another less invasive solution. Discuss with a plastic surgeon. For more info: www.Bodysculptor.Com.
Liposuction. Short of liposuction, there's no way to lose fat in specific areas w/o affecting fat elsewhere. In other words, the only way to make that fat on your neck go away is to decrease fat everywhere else on your body. Before you make an appt w/your plastic surgeon, check in w/your family doc to be sure you're not describing a buffalo hump consistent w/cushing's syndrome. Look 4 stretch marks & moon face.
Fatty neck tissue. Find the cause and eliminate it (if possible). Hormone imbalances can causes this--classically this has been a sign of hyper-cortisolism. Drugs for HIV cause this--but usually you will need to take these drugs. Excessive fatty tissue and high bmi promote this, and you treat that with plenty of exercise and much less food.