I'll have to take a guess that your Fiesta is about ten years old as you did not provide the model year your car is, so my answer will probably help but may not apply exactly.
First you will need to disconnect the positive terminal on the battery. Then you need to lift the base part of the rear bench seat - locate the clips that hold it down on the front edge of the seat and you will find a small lever that unclips each of the two fixings. Lift the bench seat up and underneath you will find a round plastic cover, pop this out and you will find the fuel pump underneath.
Disconnect the wiring plug and then the fuel hose, watch out for spilled fuel and mop up any that appears.
You will then need to rotate the round metal securing ring anticlockwise by a few degrees until it unlatches from its retaining lugs. Remove the metal ring and the white fuel pump assembly will lift out of the fuel tank. Replace with the new part and follow previous steps in reverse and the job is done. Try to avoid any chance of creating a spark whilst you are working with the open fuel tank!
If you find the description above does not apply to your model, reply with more detail on what exact Fiesta you are asking about and I will see if I can help.