No, you don't clean the U/G filter.
The closest you need to do is put a syphon hose, about the size of the uplift, and syphon from the uplift tube, with the air off.
Remember to turn it on again.
This systen pulls out most of the fine crud that has filtered down thru the gravel then the filter plate onto the floor of the tank.
To clean above the filter is a different story.
You use a gravel vac for that, this pulls out the excess loose detrius from the gravel
Once a week/ fortnight job, and for a 600x300x300 tank about 11/2 buckets of water.