1- Navigate to Mods> x64v.rpf> models> cdimages> streamedpeds_mp.rpf> (You can rename the files to whatever ped you want to replace) 
2- Delete all of the old files and add the new files. 

For ymt and yft's: 
1- Navigate to Mods> x64v.rpf> models> cdimages> streamedpeds_mp.rpf and replace the files (You can rename the files to whatever ped you want to replace)