Spoon refried beans into a small microwave-safe bowl and stir in 2 tablespoons water. Microwave on high until hot and spreadable, 1 to 2 minutes.
Heat a large skillet over medium-high heat. Cook and stir ground beef in the hot skillet until browned and crumbly, about 5 minutes. Drain and discard grease. Stir salsa into beef and cook until heated through, 2 to 3 minutes.
Spread tostada shells onto a baking sheet. Spoon 2 tablespoons of refried beans onto middle of each tostada and spread outward. Cover with cooked beef mixture and top with shredded cheese.
Bake in the preheated oven until cheese is melted, about 7 minutes.