What make and model filter do you have? Is it possible somewhere along the way the wrong type of sand got in the filter? Play sand is too fine and will find its way to the pool. In any case, back to the original question. Depends on the style of the filter but usually the top, which is usually a multiport valve assembly, is removed and that exposes the inside. Once the filter is drained a shop vac is the best way to get the sand out safely. Going to be a slow process as you have to dump the vac many times. You don't want to tip the filter, assuming you could, as that may damage the laterals at the bottom. On my Hayward the laterals are on pivoting joints and fold up like an upside down umbrella and the whole assembly is then lifted through the top hole.
Al