It means to change the music. Could be going from something fast to slow or one style of music to another

Anytime you 'switch something up' you are making changes to something. In this case the DJ is going to change the music. Below are some other example sentences using the same phrase: 1. She wanted to work with animals for a long time and decided to switch up careers later in life. 2. To switch it up, let's go out for dinner tonight instead of eating in. 3. He didn't like how they switched up the main actors in the last movie. Also, if you're interested in taking English lessons with a professional teacher, I currently have some openings in my schedule! It would be an honor to work with you. :-) https://teach.italki.com/teacher/5129645