Is it possible to change your somatotype from endomorph to mesomorph?

YES.: An"endomorph" somatotype is dominated by excess body fat, while a "mesomorph" somatotype is dominated by muscle. While you can't change your basic skeletal (bone) structure, a combination of a lower calorie diet (which decreases body fat) and a well-structured weight training program (which increases muscle mass) can shift you from predominantly endomorph to predominantly mesomorph.