Drag the audio file into the audio editing program. This imports the audio file and prepares it for editing.
Place your cursor on the approximate moment in the audio file you would like to begin the slowdown. Click on the waveform in the audio file and drag to the point where you would like to stop the slowdown. You will select the exact seconds later.
Select the "Effect" menu or equivalent option in your audio editing program. In the drop-down menu, select "Change Tempo." A dialog box will appear asking you to enter in the exact time you want to begin the slowdown and where the slowdown should end. Enter in the exact time in seconds.
Enter in the percentage that you would like the audio file to slow down in the "Percent Change" option of the "Change Tempo" dialog box. A negative number will slow the song down, while a positive number will speed it up.