I am replacing a water tank and I need to reprime the pump. How is this accomplished?

To prime a pump, look on top of the impeller housing and find a threaded 1/4" hole with a plug. This could also gave a pressure gage installed in the threaded hole. No matter, turn the pump off at the breaker box and remove whatever's in there and fill the impeller housing with water until it runs out of the hole replace the plug or gage and turn the pump back on. If you can't fill the housing up to the top that would indicate a faulty check valve. But before you prime the pump you should calibrate your new bladder tank. Let me explain the function of a bladder tank. Pressure and bladder tanks provide a air cushion that produces water pressure in your system so your pump doesn't kick on every time you draw a glass of water. A bladder tank has a neoprene membrane in the center. As the pump puts water into the bottom half of the bladder tank the membrane compresses the air in the top half. When the pressure reaches the cut off point in the control box the pump shuts down. However you still have between 40 to 50 pounds of pressure pushing the water out to your system. As the pressure drops in the tank when you make a draw the pump kicks on and everything starts over again. The bladder tank setting MUST be made with the pump off and the system bled down by opening a cold water faucet after you shut the system down at the breaker box.
Your air setting on the tank should be 2 pounds UNDER the cut in point in the pressure control box. If your pressure control has a 20 PSI cut in and 40 PSI cut out the tank pressure should read 18 PSI. This should be done with the pump off and no pressure in the system. Good luck and thank you for rating my answer, Tom