Step 3. Slide the outside sash to the bottom of the window. Remove and discard the parting beads. Remove the outside sash in the same manner as the inside sash. Don't remove the exterior stop from the window jambs. These stops will retain the replacement window when you install it.tep 1. To find the width, measure from the inside of the jamb on one side of the window to the inside of the jamb on the other side of the window. Measure at the bottom, middle and top of the window. Use the narrowest measurement for the width of your replacement window.