Download a beat maker or audio editing software. Most free beat makers will be demo versions, so you'll have limited customization capabilities. You can download a full version of Audacity (link below), a free audio editing program.
Upload your beat to Audacity. Simply click on the song file and drag it onto the Audacity icon or file name. You can also open Audacity, go to "File" and then "Open," and manually navigate to the song file.
Highlight the part of the beat you want to customize. Once you upload the beat to Audacity, it will be displayed on a frequency bar that you can toy with.
Click on "Effect" at the top of the window and choose the effect you want to add to the beat. You can also cut and copy parts of the beat. Use "CTRL" and "Z" to undo an edit. If you're using a Mac, use the "Apple" button instead of control.
Save your file by going to "File" and clicking on "Save Project." You can also click "Export As MP3" to make an MP3 file. This is your best option because most programs won't be able to open Audacity files.