The answer to 'how to fix water heater' is to first determine the type of issue you're experiencing, such as a leaking tank, not heating properly, or making strange noises. Depending on the problem, you may be able to fix it yourself with some basic troubleshooting and repairs. However, for more complex issues or if you're not comfortable working with plumbing and electrical systems, it's best to call in a professional HVAC technician.
If the issue is a leaking tank, you'll need to turn off the power or fuel supply, drain the tank, and inspect for any cracks or corrosion. You may be able to patch a small leak, but a severely damaged tank will likely need to be replaced. For problems with heating or strange noises, you'll want to check the thermostat, heating element, and other components to identify and address the root cause.