Peel the sweet potatoes and cut them into large (2-inch) pieces. Add the 1 ½ cups water to the bottom of the Instant Pot inner pot. Place a steamer basket in the pot. Put the sweet potatoes in the steamer basket. (If you do not have a steamer basket you can set the sweet potatoes on top of the metal trivet that comes with most Instant Pot models.)
Close the Instant Pot lid and turn the steam release valve to the sealing position. Set the cook time to 15 minutes at high pressure using the Pressure Cook or Manual function.
The Instant Pot will take about 15 minutes to reach pressure and then the cook time will begin counting down. When the cook time ends, quick release the pressure by carefully turning the steam release valve to the venting position (I do this with the handle of a long spoon). Once all of the steam has escaped and the pin drops down, carefully open the Instant Pot lid.
Use a spoon to transfer the potatoes to a bowl. Wash and dry the Instant Pot.
Add sweet potatoes back to the empty Instant Pot. Mash the sweet potatoes using a potato masher, hand mixer or immersion blender. Then stir in the butter, maple syrup, salt and pepper. Finally, stir in the warm milk, adding more milk as needed to reach your desired texture. For the creamiest, smoothest mashed sweet potatoes, I like to first mash them with a potato masher or hand mixer. Then, after mixing in the other ingredients, I use an immersion blender to get them really smooth and creamy.
Taste and adjust the seasonings as needed. Serve.