How to extend my washing machine standpipe?
That should be easy. Start by figuring out how much additional height you need. Use a pipe wrench to loosen the faceted collar at the top of the trap, and the standpipe should just pull up and out of that fitting. Take it to a store and tell them you need a piece the same size but however-much longer. You may have to get something a bit longer than that and have the store cut it down, or use a hacksaw to cut it down yourself. Slide the new pipe into place, and use the wrench to tighten the collar of the slip-joint again to seal the connection.

(Depending on how it goes, you may need to repack the slip-joint to get a good seal. That isn't difficult either -- remove the ring entirely, wrap some rope packing around the pipe just above the junction (or install a new packing washer, depending on the age of the hardware), slip the ring back over the tube and tighten it onto the threads; doing so compresses the packing and makes the joint waterproof.)