Make sure the oven is off and cool (turn off the circuit breaker). Wear a leather glove to protect your hand. Depending on your model, there may be a glass shield held by a metal holder over the lamp. In the case of a unit with this holder, press on one side to release the shield and remove it. Remove the bulb by turning it counter-clockwise. Replace with a 40 W appliance bulb (turn clockwise). Replace the glass shield and the holder. (On some oven models, the shield will twist on and off like a bulb.)
There are several models of Kenmore 47783 Ovens; manuals are available here: http://www.managemylife.com/mmh/owner_manuals/search?query=Kenmore+47783
I hope this helps.
(please note: never operate the oven without the shield in place if your oven has one.)