If they are "nice", why renovate them? ;-) Otherwise - by sanding the existing varnish (and the surface layer of wood) and repainting *) with a quality varnish, you will of course extend their service life and improve their appearance. If there are no significant gaps between the individual parquets, I do not recommend sealing them, the sealant will crack and crumble over time (which on the one hand does not look good, but mainly the created joints will allow the ingress of water and dirt). If parquets fall (and it bothers), it is better to call in experts and have a price proposal prepared in advance (it is usually necessary to ´ tear off the entire parquet covering, sand on both sides and re-lay - which is labor-intensive and therefore expensive ). *) instead of varnishing, the surface can be treated with wax or oil - this will enhance the beauty of the wood, but this treatment is less resistant (eg water) and needs to be renewed at regular intervals