Use a video editor with audio noise reduction tools
Isolate the audio track from the video
Apply a noise reduction or denoise filter
Capture a noise profile from a silent section
Reduce the noise amount gradually
Use a high-pass filter to remove low-frequency rumble
Use a low-pass filter to reduce high-frequency hiss
Apply a noise gate to cut background sound during pauses
Use an equalizer to lower problematic frequencies
Normalize or compress audio after noise reduction
Remove hum with a notch filter if needed
Re-record audio in a quieter environment if possible
Use a directional microphone for cleaner future recordings
Export the cleaned audio and replace the original track
Test different settings and compare results before final export