I am about to change fuel filter, could anyone tell me where is it located and also any suggestion to do it by DIY.
Thanks!!

I've not done it, but watch my tech do it! Located under car on right side in the back near differential in a metal 'shroud'. There's a pump and the filter.

Not complicated but requires 'diligence' so that gas line doesn't leak after job completion.

A service manual would help if you've not done it before. But then again, it's not brain surgery!

Fuel filter replacement
 

I just replaced the fuel filter on my 1995 S420. Your car is the same, and it is easy to do. I used a Knecht filter (made in Germany), purchased from Randy Steele at E.P.S.
Block the front wheels of your car, raise the rear end and place onto jackstands (I use heavy 6-ton jackstands because they have nice wide bases and are more stable than the 3-ton units). Remove the plastic shroud that covers the fuel pump and filter. It is just ahead of the right-rear halfshaft. I use a 1/4-inch drive ratchet wrench and a couple of extension bars to remove the 3 plastic nuts that hold the cover on. 10 mm size, if I remember correctly. After dropping the cover, you'll see the fuel filter and electric fuel pump held in a metal bracket with a single screw. Place an oil catch pan under the filter, and remove the 2 rubber hose clamps and hoses from the filter. Loosen the filter clamp screw, and as you slide the filter out of the clamp, look to see which way the Direction of Flow arrow points, so you'll know which way to position the new filter. (Flow goes from front of car towards fuel tank if you forget to do this step). Slide the new filter in, tighten the clamp screw, and slide the 2 hoses on and tighten the hose clamps firmly, but don't overdo it or you'll cut into the hoses. Start the car and run it for a minute or 2 to check for leaks. Re-install the plastic cover and you're done.

Have fun,
Mike

I was going to change mine but one the 1999s they have crimped hoses.What size replacement(and where to find) would replace them. located clamps here at pelican parts