Visit the manufacturer's website to determine if upgrades are available. If they are, purchase the required part and take it to a music store for installation. Alternatively, follow the instructions included with the part to install the upgraded hardware yourself.
Turn the synthesizer over and look for the tuning wheel located beneath the keyboard. Better synthesizers will have wheels for each key, while the less expensive synthesizers will only have one master tuning wheel.
Adjust the tension of the synthesizer keys by turning the wheels clockwise to increase pitch, and counterclockwise to decrease the pitch. Set your tuner to the pitch you want to tune each key to, and match pitch with your tuner.