Oh okay.
So…
1. Find the song you want to use
2. Check if there’s a download link (if yes, move on. If not, R.I.P.)
3. Download song (make sure it’s an MP3 file)
4. Open files and press Windows + R and type Appdata
5. Find and open the Geometry Dash folder
6. Replace the song you want to replace with the NONG (make sure it uses the same song ID as the song you’ve replaced)
7. Have fun :)