Install a music editing program. In order to change the tempo of a pre-recorded song or sample, you will need commercial software, but the program need not cost a fortune. In fact, you can even use free and open-source programs. Examples include Audacity, Power Sound Editor and QTractor.
If it is on a CD, copy the song onto your computer. If you already have the file on your computer, skip to the next step. Otherwise, you can copy the file by inserting the CD into your disc drive and selecting the "Rip" or "Import" option on a music media program like Media Monkey, iTunes or Windows Media Player.
Open your audio editing software. To import your song, click "File > Import" on your menu bar. In rare cases, your program may use different wording, such as "Import MP3" or "Import Audio File." Use the navigation window to select your beat file.
Locate the "Tempo" indicator on your audio editor window. It should appear as a small box, sometimes indicated by the letters "BPM" (Beats Per Minute) or the word "Tempo." In some cases, you may just see the number "120," a standard default tempo. For example, in QTractor, you can find it near the top center of the window. If your software does not feature a "Tempo" indicator, proceed to the next step.
Click your "Effects" menu on your menu bar. Some programs, like Audacity and Power Sound Editor, place tempo editing options under the list of effects. Under your "Effects" menu, select the option that reads "Speed" or "Tempo." A new window will appear on your screen. If using a different program with no "Effects" menu, look for a menu that reads "I/O" (Input/Output) or "Track Options" on your project window, mixer or menu bar.
Reduce the number in the tempo box to slow down the tempo. Whether using a "Tempo" box on the project window or a "Speed" window under your "Effects" menu, the process is the same. For example, if the tempo currently reads "120," you might reduce it to "105" to reduce the tempo by 15 beats per minute.
Click the "Play" button your audio editor window to hear the changes. If the audio file now sounds distorted, raise the tempo slightly to reduce the intensity of the change. If you reduce the tempo by a large amount, the quality of the recording will suffer.
Click "File > Save" to save your changes when you feel satisfied, and click (File > Export) to save the project as a music file. If you do not see an "Export" option under the "File" menu, look for an option that reads "Save as MP3" or "Bounce."