Spot reduction, the idea that we can lose fat from a certain place on the body, is a myth. That doesn't stop the "fitness" industry from making millions of dollars selling crap to ppl who do want to believe it, but it's a myth. It doesn't happen like that. We can't choose where to lose fat.
The method for losing fat from your back is the same as the method for losing fat anywhere. Maintaining a caloric deficit through diet and/or exercise. Your body determines where fat is burned and stored. Generally, the belly is the last to go, which is why having a flat belly or abs is the Holy Grail of fitness.
You just gotta eat better, and exercise some more. Try a calorie counter like myfitnesspal.