A 60W LED bulb is likely labeled as “60W replacement” on the package—that’s because it provides about the same light output as a 60W incandescent bulb. In reality, it likely uses less energy—usually 15 watts or less. If that’s the case, then it’s perfectly fine to put a 60W LED bulb in a 40W socket. Your best bet is to read the package carefully. It will tell you the LED bulb’s actual wattage, and as long as it’s lower than the fixture, the bulb will be safe to use.