The garage door header is the large piece of wood at the top of the garage, which supports the roof of the garage and any living space above it. If you notice that your garage door header is starting to sag, or has developed cracks, then you may have a serious structural problem. The best way to fix this problem is to replace the header.
Step 1 - Remove the Garage Door Header
Take down any trim, or any external pieces of wood, which are not part of the garage door header. Support the ceiling of the garage using some lumber. Take out the header by sawing through the center piece. Remove it, and then take out the pieces at either end. You will need to remove the whole piece of the header, so it is best to have someone to support the piece while you are pulling it out.
Step 2 - Replace the Header
Push the new header into place. This header should be an inch and a half higher than the garage door, and 9 inches wider than the door. Take some 2x4's, and use these to add 3 rows of supporting frames around the sides of the door, and just below the header. These will help to support the new garage door header.