Listen to your track to decide what you need to adjust. If your track sounds muddy, you should cut, or lower, the bass (low-pitched) frequencies and boost the treble (high-pitched) frequencies. If it is too tinny- or scratchy-sounding, cut the treble and boost the bass.
Locate the EQ controls on your mixer. Most mixers, including computer mixing software, use knobs (also called pots) instead of sliders. The knobs that control a specific track will run in a straight line vertically on the mixer. Often the higher knobs control the treble frequencies, while the lower knobs control the bass frequencies. These are generally labeled "High", "Mid" and "Low".
Set all of your knobs to the center position. This is the starting point, often marked with a "0" or "U". This will let you hear the recorded sound exactly as it is, with no changes.
Play the track. If you would like to boost the bass frequencies, find the knob marked "Low". As you listen to the track, slowly and carefully turn the knob to the right. The bass frequencies of the track will become more prominent. To decrease the bass frequencies, turn the knob to the left.
Repeat the process for the mid and high levels, usually indicated with "Mid" and "High" labels. Turning them to the right will boost the frequencies, while turning them to the left will cut the frequencies. Experiment with them to find settings that make your track sound bright and clean.