I made this to use with mpclothes but if you don't want to you can replace the bun hairstyle already in the game located in:

update\x64\dlcpacks\mpgunrunning\dlc.rpf\x64\models\cdimages\mpgunrunning_female.rpf\mp_f_freemode_01_mp_f_gunrunning_hair_01

Enjoy.