Re: how do i backwash
Since you don't have the instructions, you probably do NOT know that before you clean your filter (as Dennis described) you need to block off the two lines to the filter or you will have a MESS! Intex should have given you something--older ones had a plug that looks sort of like half a black golfball with a flange around the middle. You'd jam them in each hole in the pool (properly called the "Drain" and the "Return") before you open the filter.
If you don't have them or something similar, you'll need to go to a pool store and get an expanding plug. It looks like a high school chem class rubber cork, with a screw and a wingnut to expand it. You'll use those to seal off the lines to the pump first. They come in different sizes but you'll probably need the one that fits the 1.25" hose.
I don't really know how Intex does the metal pools--my Intex was a donut from 7 years ago.
16'x40' rectangle 19K gal AG vinyl pool; ; Hayward T210 sand filter and 2 Speed Superpump 1hp pump; Solar heated. Test kit: PS 233; PF:6.3; Autopilot SWCG