Instructions
Click the treasure chest icon in your monster's room to open the chest and display the items within it as cards along the floor.
Align your cursor above the right scroll arrow to go to a section of the room with a window on the wall.
Click the "Sunken Window" card without releasing the mouse key or lifting your fingertip and then drag the window to the location on the wall occupied by the existing window.
Release the key or lift your fingertip from the screen. The two windows will switch and the Sunken Window will appear on the wall.