Cinquecento/Seicento 1.4 engine into a cinq (useful info and links)

This is covered very well in the haynes, but:

Make sure the battery is not connected
If you already have the back seats out, brilliant. This is easier.
You need to gain access to the top metal cover above the fuel tanks from under the back seat so will need to remove.
Undo the fuel cap to release pressure, just in case
Undo the 4 screws holding the metal plate in place
Unclip the 2 wiring connectors
Undo the nut on the retaining bracket
Pull out the supply and return (a bit off tissue handy is good at it will drip)
Using 2 beefy flathead screwdrivers insert them into the nothes. Then use a 3rd in between them to rotate the plastic anticlockwise if that makes sense.

Take out the sender
Let this drain as it holds quite alot of fuel
Unplug the pump wiring connector
Undo the fuel pump bolts, being careful not to drop them into the tank in a second
Lift out the pump (let it drain again)
Replace with new pump and refit as you took it out.
The Fiat barchetta pump i used was slightly shorter and thinner, this meant i had to make a rubber seal around the pump to hold it tightly in place. Fairly obvious when you have the pumps side by side.
Reconnect all wiring etc and then battery if you really must, but personally i would leave it off.