Ignore my advice frictioncircle, the i3 PDC sensors are nothing like the X5 in terms of rotate to fit. I had to fit a second hand sensor tonight to clear a parking PDC issue I've had for some time.
Jack up the car, pop off the front wheel, put an axle stand under the subframe chassis just in case.
Remove the 8mm screws and one 10mm nylon hex nut from the wheel liner and remove. Interestingly the wheel liner is in two parts, you only need to remove the section closest to the fender. It's easier out of the way. With full access you can now see the PDC sensors, especially the outer ones, not to be confused with PMA sensors on the wings. For the outer ones there's no need to drop the cover underneath.
The sensor is held in place with a moulded bracket in 4 places. Pressing on the PDC, and unclipping each plastic arm in place you can remove. Clean the aperture with a little WD40 and refit. Check the wiring to the sensor, it's one of these lift clip to disconnect. Pictures here.
2015 i3 REx, 60ah, solar orange. Reversing camera, leather interior, sunroof, park assist, professional navigation.