White blood cells change when you have an infection, are undergoing chemo, take certain meds, smoking, certain diseases, maybe even have serious stress for a long time.
Sometimes we can't control what happens to our WBC, but generally speaking, living a healthy lifestyle and avoiding stress as much as one can can help.
It's not something you need to really worry about unless you get sick.
https://www.mayoclinic.org/symptoms/high-white-blood-cell-count/basics/causes/sym-20050611