Let's talk about repairing broken glass in an aluminum frame single hung window. If it's the lower sash pane that is broken, it must be removed from the inside. You are going to have one of three different mechanisms that hold the lower sash up when you slide it open. If you can't see any mechanisms on the sides, then you have a block and tackle system consisting of a string and spring assembly. Find the thin metal clips in the side jambs just above the sash. Pull the bottom of the clip out using a screwdriver or your fingernail. Do that on both sides. Then remove any rubber stops at the very top of the window. Raise the window as high as it will go. The block and tackle assemblies will get snagged in the metal clips, allowing you to remove the window sash. You would replace the glass using the same method described in our article about sliding window repairs. Once you have the new glass installed, install the window panel in the reverse order that you removed it. Close the window and push the metal clips back. Install the rubber stops at the top.
If you have a mechanism across the top of the window with a string coming down each side and screwed into the top corners of the window sash, you need to remove the screws holding the strings in place. But before you remove the screws, you need to remove one of the black plastic pieces that cover the side jamb. Raise the window all the way up, then put a flat screwdriver at the very bottom of the plastic piece and pull outward until you can grab it with your fingers. Slide the plastic out. Now remove the screws holding the strings. Be sure to hold the string in one hand while removing the screw, because the string is under tension. After removing the screw, let the string slowly go back up. Pull the panel to the side that you removed the black plastic piece, and remove the panel. Remember, two of your corner screws will be removed at this point, and the proper way to remove the frame from the glass is to remove opposite corner screws. So, you should put one of the string screws back in and remove the corner screw opposite the removed string screw. Then, when you install the new glass, remove the string screw and install the strings. Raise the window up and install the black plastic piece by sliding it up between the frame and side jamb.
The final mechanism will be a spiral metal rod. If you have this type, i suggest you have it done by a professional. If you aren't careful with these, you can wind up having to replace the spiral rods. Now, if the broken pane is the upper sash, more than likely it will be a fixed panel. You can take these out without messing with the sliding panel. Unlock the window and raise the lower sash a few inches. Remove the screen, then remove the two screws holding the horizontal bar on place. Tap the bar down in one corner using a hammer and screwdriver. The bar will come off. Then you can grab the bottom of the upper sash and wiggle as you pull the sash downward. Once you have pulled it down several inches, you should be able to grab the top and bottom of the sash and pull to the left or right until the opposite side comes out. Then you can swing the panel out.
Once the new glass is installed, you can put the upper sash back by reversing the removal procedure. Tap the bottom upward with a hammer or rubber mallet, ensuring that the sash is fully seated in the top of the frame. Then, put the horizontal mulling bar back in place and tap it on the sash frame. Install the screws and screen. You're done.