This is an easy one!
First, if it's 3+ years old, replace the sand while you're at it (might as well get Zeobrite as a replacement).
Remove multiport valve by undoing all bolts where it joins the filter.
If the sand or medium is fairly new, you may want to save it by removing it into a clean container or containers, some will have to be washed out with the filter on it's side to get it all out.
NOTE: Wet medium is going to be tougher to put back in than dry medium.
Once the sand's gone, you'll see the laterals pointing out like a star in the bottom, you can lift the tube some, reach in and unscrew them one by one, inspect them and replace the broken ones, I believe they're relatively cheap and you may want to replace them all since they get brittle with time.
Since you know your filter brand and size you should be able to pre buy them before starting the job so the system is only down for an hour or so.
If you don't want to replace them all (usually 6 pcs - your diagram may be available on-line), at least buy 2 or 3, that way if you have one left over it can be a spare for when you have to do it again.
Put everything back together in reverse order.