Hook up a water pump capable of handling sand to the well if you don't already have one installed. Follow all instructions for the pump because installation varies depending on the make and model.
Insert into the well a garden hose that is hooked up to a water supply and can generate high pressure. The end of the hose needs to be touching the bottom of the well.
Turn on the water hose at its highest setting to maximize the water pressure. The pressure goes into the bottom of the well and forces sand particles up off of the well floor and into the water.
Pump the water out of the well until you no longer have sand coming out with it.
Turn off the water valve for the garden hose and pull the hose out.