OpenIV (Edit Mode):
1-x64u.rpf/levels/gta5/_hills/country_06/cs6_03.rpf
2-Find "cs6_03_flag.ytd" and replace it with the "cs6_03_flag.ytd inside" the "GTA 5 Rebel Turkish Flag" folder.
3-Enjoy!

(Desteğinden dolayı Batuhan Çelik'e teşekkürler...)