Open Audacity and click on the "File" button at the upper left corner of the screen. Click "Open" from the pop-up menu and locate the song you wish to slow down. Double click on the song to open it.
Highlight the section of the song you want to slow down. You can do this by clicking on a section of the music with your mouse and dragging the box that appears across the selection you want to slow down. If you want to slow down the entire song, you can select everything by holding the control key while pressing the "A" key.
Click on the "Effect" button at the top of the screen. Choose "Change Speed" from the menu that pops up.
Use the slider or the percent change box to choose how much you want to slow the song down. You can preview your speed change by clicking the "Preview" button at the bottom of the box. Once you are happy with the amount of slowness, click "OK" to prompt Audacity to slow the selected portion of the song.