There are only a few reasons why you would need to replace your intake manifold. Since it is not a moving part, it is not very susceptible to damage. In actuality, most will fail due to a design flaw or in concert with a failure of another part rather than from overuse. The most common signs your intake manifold needs replacement is when you can spot a leak. Many are either cast aluminum or hard plastic, and sometimes integrated with the thermostat or coolant outlet. Temperatures tend to reach near 200 degrees, and a coolant leak up front or a vacuum leak in the plenum are the most frequent problems. Drivers should regularly take note of their engine compartments for visual inconsistencies, and monitor their gauges for warnings. A coolant leak in the intake manifold will cause temperatures to increase, and you'll see a slight decrease in coolant levels that simply topping-off won't fix. And a vacuum leak in the intake plenum will cause your engine to run lean, starving it of power. In addition to these issues, a cracked or broken intake manifold gasket could cause your engine to run hot or lean as well. This should not be ignored, and fixed as soon as possible.
Your intake manifold sits atop your engine, and evenly distributes your air or air/fuel mix (depending on the type of fuel delivery system) to each cylinder via a network of ports. On older engines, the carburetor would be mounted to the top of the cast iron or aluminum manifold. It would draw air and fuel into the chamber, tumble it so that it mixes well, and send it to whichever cylinder was on the intake stroke. After fuel injection became the norm, most intake manifolds simply held a volume of air in a plastic upper plenum and delivered it to the cylinder where it would receive a fine mist injection of liquid fuel immediately before compression and combustion. An intake manifold gasket separates the manifold from the cylinder head, providing a tight seal in order to maintain optimum levels of compression, turbulence, and temperature.
Depending on the age of your car, your engine bay can be very simple or very complicated. Generally speaking, the newer the car is then the harder it will be to perform a large job like replacing your intake manifold. Some accessories will take advantage of the partial vacuum in your intake plenum to actuate or power them. That would complicate the replacement procedure because of all the extra hoses and connections. Be careful before attempting this process.