Lght fixtures have a rating for the particular size bulbs (based on the wattage) you should use in that fixture.
As long as the mounting base (socket) is the same size and type, you can use an LED bulb in an existing fixture. If the mounting base isn't the same size and type, the LED bulb will not fit the socket.
You should never use a bulb with a higher wattage than what is recommended for the fixture.
Don't forget that watts are not the same as the amount of light emitted (lumens). Watts are a measure of the electric power. LED bulbs have much lower wattage than incandescent bulbs, so it's important to know the light output (in lumens) for the bulb you're replacing.
