Allow the paint to cure. Depending on the type of paint and the amount used to cover the surface, this can take anywhere from 10 to more than 30 days. For best results, wait a month before attempting to remove the brush marks.
Sand the brush marks away. Depending on the size of the marks, you might elect to use a sanding tool or simply rub the area manually with sandpaper. Sand the marks repeatedly until the surface is smooth to the touch.
Repaint the area you just sanded. Depending on how large it is, you may prefer to repaint the entire surface again. Once the paint dries, the brush marks will be gone.