...By default, it will be saved with 2 audio tracks of singer + melody.
If you want to save only the melody without the singer's voice, you have to select the No Vocal option.
To save the output file, click Save Folder and choose the location you want to save (Default: Desktop).
If you are sure of the above preparations, you can change the file you want to change by holding down the mouse and dragging it onto the Drag & Drop Input File. (No internet needed)
You can also change it by clicking Select File.