how do i replace the oil pump on a 2001 ford fiesta 1.8 diesel?

the oil light on my fiesta keeps coming on so i replaced the oil pressure sensor to see if that was the problem but it still comes on. when i accelarate it flickers the oil light but it is usually on all the time. the oil level hasnt dropped and i can see oil in the rocker cover. do i need to replace the oil pump and if so how?