The answer to 'how to maintain hedge' is to regularly trim and shape your hedges. This involves using hedge trimmers or loppers to cut back the sides and top of the hedge to maintain its desired shape and size. Trimming should be done every 4-6 weeks during the growing season, and less frequently in the winter months.
It's important to trim hedges in the right season - for most types, the best time is in late spring or early summer when the new growth is lush. Avoid trimming in the hottest part of summer or during drought conditions, as this can damage the plants. Always use sharp, clean tools and make clean, even cuts to avoid damaging the hedge.