For a leather sofa, use a clean, dry, nylon bristle brush to remove as much of the mildew as possible. Then, using a soft cloth dipped in sudsy water, scrub the area. Allow the sofa to air-dry out of the sunlight. For an upholstered couch, make a solution of hot, soapy water and use a soft cloth to wash the area clean. Allow to air-dry. Do not use bleach as it may spot the fabric.