"To change the mix" means to alter or modify the combination or assortment of something. It refers to making changes in the composition or proportions of different elements within a group or system. Full definition
Such a program could also affect public schools by changing the mix of students who attend them. (educationnext.org)
Remember to change each mixed number to an improper fraction, then multiply by the reciprocal and simplify where you can. (thoughtco.com)
I'll be * very * interested to see what happens to the reputation of blogs which change their mix to become fairly pure campaigning platforms. (nextleft.org)