Sorry, but a little more information as to exactly what product/version you're using and what you're trying to accomplish may be helpful to proplery guide you through this procedure.

When you're saying this is about a "song in your music folder", I assume this is about an existing audio file (not about a Music Maker project). In this case, the only way the tempo could be changed is via the time-stretching algorithm (especially if you don't want the pitch to change at the same time). This is generally possible in Music Maker, again depending on the version you're using. If this feature is unlocked, you can access it by right-clicking on the imported audio file/object and going to "Pitch & Tempo" => "Timestretch & Pitchshift" in the context menu. Simply adjusting the project tempo (BPM) will also result in the change of the tempo of the imported audio file (unless you have specifically disabled this in the program settings). However, it is still the time-stretching algorithm that will be used for this.