There are two ways about cleaning your spa filters. Depending on how
dirty they are.
Note: You only have to replace your filters if they start to fray.
Meaning fall apart. If they are just filthy then they can be cleaned.

For a mild cleaning job thats quick you need to find a purple degreaser
like 409 or visit your local spa store and ask for a filter degreaser.
Note: They use the same formula for cleaning sand and de filters.

If not already in a spray bottle, but some in one, and generously spray
your spa filters outside. Once they are completely covered with the
formula, let them sit for about 15 to 20 minutes. Then take your hose
and rinse them like you've never rinsed anything before
Note: If you do not rinse them very well, when you put them back in
your tub, you will have an uncontrollable bubble bath on your hands.

Now if you have a deep (hardcore) clean then you must buy muriatic
acid.
Note: yes this is real acid and will burn you so be very careful,
always wear gloves.
dilute acid with water 4 to 1, one being acid. and soak your filters
in the acid solution for about a half a day. Again when you remove
them rinse them like crazy. You wont have foam if you don't but it will
drastically drop the pH in your water.

For more information visit
http://www.swimuniversity.com
http://www.niagarapool.com