Detach both T25 screws found on top of the A4's headlight, with a T25 torx screwdriver in a counterclockwise motion.
Loosen the two screws located at the sides of the headlight of the A4 using a T25 torx screwdriver, without removing the screws. Connect the T25 torx screwdriver to an extension piece to receive a closer access of the sides of the headlight.
Slide out the headlight and pull the metal clips located behind the headlight to detach it from the wiring.
Clasp and remove the back cover of the headlight and push the metal clips found at the back of the light bulb to detach the wiring. Pull the light bulb out with a cloth to avoid dirtying the bulb and slide the new bulb in.
Repeat the steps in reverse on the A4 to assemble all the components that were removed.