Removing cool season grasses from Bermuda lawns is not an easy job. If temperature variations between your cold seasons and hot seasons are extreme enough, the problem usually resolves itself by the grasses dying off naturally. If temperature variations are not that extreme, however, you will need to physically remove the cool season grasses at the end of their growing season (late fall or early spring). This removal can cause your lawn to look rather ragged for a while, and it is recommended that additional Bermuda sprigs (stolons) be planted in bald spots.
Mow your cool season grass low, setting your lawnmower to approximately 1 inch. Use a bag to catch as many of the cuttings as possible, then discard.
Run a power dethatcher over your lawn in one direction, then run it across your lawn again in a cross pattern. Attempt to pull up as much of the cool season grass as possible. Bag and throw the grass away as the seeds in the thatch are not good to compost.
Use a small spade or trowel to dig holes and plant new sprigs of Bermuda grass (pieces of Bermuda with roots attached) in areas left bare by dethatching. The more sprigs you plant, the quicker the Bermuda will fill in and make your lawn look lush as warm weather arrives.
Spread 1/4 inch of organic manure over the surface of your lawn and water well, putting at least an inch of water into your soil. This could take as long as an hour.
Keep the lawn moist but not soggy until the new Bermuda sprigs shows signs of growth. Water the lawn once or twice a week after that, following your normal watering schedule. Do not let the ground become completely dry between waterings.