How I can increase the pressure from my shallow water jet pump?

Assuming everything is working properly, you do that at the switch. This site has a very nice description of how it's done. It's not difficult. The only thing I would add is this: Quarter turns of the nut don't accomplish much. Think three or four full turns at a time as you go though the trial and error process of adjusting.

How to adjust water pump pressure, pump cut-on pressure and pump cut-off pressure - private pump and well system do-it-yourself repairs (http://www.inspect-ny.com/water/WaterPumpAdjust.htm)