Decorative window mullions, often called window grills, are replaced by determining the specific type required, finding the exact size needed and installation per the mullion manufacturer's recommendations. Note that many residential windows are designed to accept mullions from the original window manufacturer.
First, determine the manufacturer of the window and check with that manufacturer for mullion options. This is often the easiest overall and provides the best chance to match other mullions in the home.
If an OEM product is not an option or is not desired, determine how the mullions being replaced are attached. The most common attachment method is via clips or pins at the frame of the window pane. These are easily removed by disengaging the pin or clip. This design also facilitates easy cleaning. The mullions may be permanently attached to the glass or mounted in between panes of a multiple-pane window. These types should be referred to the window manufacturer or a qualified repair professional.
If the replacement job is within one's ability, measurement is the most important step. Always refer to the measurement guidelines offered by the mullion manufacturer. Many suggest measuring the dimensions of each pane, not the total area of a double- or triple- hung window. Precise measurement provides a good fit and results that appear professional.
Choice of style and material is likely dictated by matching other windows in the home. Material options include wood, fiberglass, vinyl and aluminum. A wide variety of pre-finished styles and sizes, as well as many kits allowing for custom sizing, are readily available at most home improvement retail centers.