Snip off any dead leaves or stems immediately when you see them to prevent the plant from decaying in the water. Sever the dead or damaged leaf or stem down at the base of the plant to prevent partial leaves or stems from marring the appearance of the plant.
Trim the top of the plant that has grown taller than the waterline and may be blocking light by cutting just above a leaf joint on the stem, anywhere below the water line as desired.
Thin overgrown plants to allow more light penetration, room for other plants or just better fish viewing by removing the unwanted leaves and stalks down to the crown of the plant just at or slightly below the planting gravel or other substrate.