Double-glazed windows have two panes of glass instead of one, with a small air space between them, which makes them more energy-efficient than regular single-sheet windows. If you're replacing your old double-glazed windows, and you're not using double-glazed replacement windows, you're going backward, efficiency-wise. Double-glazed replacement windows install in the same manner as other kinds.
Measure the opening of the window while the existing window is still in place, taking the measurements along the height and width. Take them from within the tracks where the window sashes sit. Order your double-glazed replacement window units using those measurements.
Remove the old windows when the new unit arrives, by prying off the interior trim border from the inside edge of the opening, then pulling the sashes straight out. Don't break the trim as you remove it.
Stand the new double-glazed window unit in the opening, bottom-first. Push the top end back into the opening until the exterior border of trim stops it.
Put a level alongside the window unit and shim it around the sides until it's level.
Drive long mounting screws through sides of the unit, outward into the surrounding frame, through the provided screw holes to secure it.
Shoot spray foam insulation into the spaces around the edge of the window unit, filling the spaces.
Nail your border trim back in place with a trim nail gun, trapping the window in place. Run caulk all around the perimeter.