1. Open the hood. 2. Remove the upper filler panel by loosening all the screws and then removing the grommets. 3. Remove the two bolts fastening the headlamp to the headlamp mounting bracket. 4. Pull the headlamp straight forward to disengage the locator pin at the outboard edge of the headlamp. 5. To replace the low-beam bulb, first remove the rubber housing cover on the rear of the lamp. 6. Then, turn the bulb socket counterclockwise to unlock the socket from the lamp housing. 7. Remove the headlamp bulb socket. 8. After removing the wiring harness connector from the headlamp bulb socket, replace the bulb assembly. Reconnect it to the wiring harness and reinstall the headlamp bulb assembly back into the headlamp housing. 9. After reinstalling the bulb and socket, reinstall the rubber cover. Make sure it fully covers the headlamp socket. 10. To replace the high-beam bulb, turn the bulb socket counterclockwise to unlock the socket from the lamp housing. Follow the instructions in Step 6 through 8 to install the new bulb. 11. Reinstall the upper filler panel by inserting the grommets, along with the screws, back into the holes.
I copied this straight from the owners manual. But the instructions really make more sense with the pictures. If you don't have the owners manual and want one, you should really have it. You can download one free here: http://www.autousermanual.com/view/cadillac/seville/