After 15 minutes you can release the pressure valve.
Stir up the beans to mix everything together. Then pour the beans into a strainer, making sure to reserve the cooking liquid. Return the beans to the Instant pot stainless steel liner. Add back about 3/4 cup of the liquid.
Using an immersion blender or a potato masher roughly mash or puree the beans completely to your liking, using more of the liquid to get it to your desired consistency. Keep in mind the refried beans will thicken as they cool. I only used 3/4 cup liquid and that gave me the cossistency like canned refried beans.