Depending on the damage to the French door, the options for repair include replacing the glass on the door, replacing the entire door, or if it is a double French door, replacing the pair. Smaller repair issues may require replacement of the weather-stripping or door hardware.
If replacing the glass, the old glass must be removed to make way for the new glass. Wearing gloves will prevent injury from sharp pieces of glass, and safety glasses will keep shards of glass away from the eyes. Cover the glass with a towel, and tap the pieces so they fall out easily. Stray pieces that cling to the putty must be removed. After all the glass is gone, remove the trim and take measurements of the open space.
After the pane is cut, dab a small amount of caulk on the setting to hold the glass in place. After the glass is tapped into place, replace the trim, and run caulk around the outside of the trim. Any visible holes may be puttied, and the trim can be painted.
Replacing the door is a matter of measuring the old door and finding a suitable replacement. If it is a double-sided French door, the new door must be matched to the old one. In this case, if the design is no longer manufactured, it's easier to replace both doors.