The answer to 'how to fix installation' is to first diagnose the issue. Common HVAC installation problems include improper ductwork sizing, refrigerant leaks, and electrical wiring errors. Start by inspecting the unit and checking for any obvious signs of damage or improper setup. If you're comfortable doing so, you can try troubleshooting basic components like the thermostat, air filters, and electrical connections.
However, for most HVAC installation issues, it's best to call a professional HVAC technician. They have the expertise and specialized tools to properly diagnose and fix any problems with your system. Attempting DIY HVAC repairs can be dangerous and may even void your warranty, so it's generally not recommended unless you have extensive HVAC experience.