Remove the Old Window Sashes. Since the replacement windows fit inside the existing window frame, you only have to remove the sashes, parting bead and inside stop from the existing window. To make the project easier and safer, install your new windows from inside your home.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.