Remove the glass from the frame. Lay a dropcloth down and break the glass up with a mallet. Pull out the stubborn pieces of glass from the frame with pliers.
Push the window up or down so the sash hangers at the top of the frame line up with the side channel's notches. Push the locking tabs on the bottom of the frame together. Pull the window frame out and down from the window.
Lay the frame flat. Pull up on the rubber gasket so it releases from around the inside of the frame. Inspect the gasket for wear. Discard worn gaskets or reuse the old gasket to fit the new glass pane.
Measure the thickness of the gasket and double that measurement. Take note of the length and width of the frame interior. Add the doubled thickness of the gasket to the width and length of the frame interior. Take that measurement to a home improvement or hardware store to purchase a new glass pane. Purchase a new gasket if the old one is damaged.
Pull the rubber gasket over the edges of the new glass pane. Cut the excess gasket off with a utility knife.
Remove the screws from the corners of the top of the frame. Slide the top of the frame off the "L" brackets inside the sides of the frame. Use caution not to twist or bend the "L" brackets when pulling off the top of the frame.
Line the sides of the glass pane up with the sides of the frame. Push the glass into the channel inside the frame. Slide the glass all the way into the frame. Set the top of the frame back in place with the "L" brackets aligned. Tighten the corner screws so the glass is secured into the frame.
Push the sides of the frame into the side channels of the window openings until the top of the frame is touching the top of the inside of the window opening. Push the lock tabs at the bottom of the frame apart to set the bottom of the frame into the window opening. Let the lock tabs snap into the side channels to secure the window.