How do you change a cartridge filter to a sand filter?

why to change it in the first placein the first place why we should chnge the sand in the filter, why can't we jus try to wash it where as it is still in place ,how ever if it is still needed to change the sand I think that can be achived by using any kind of a machine working as a hoover and that can be done through the manholes at the top of the sand filter.

In other Words, use a shop vac.....