How to replace a player's character with a new character in Horse Starters kits?

I want to use the entire setup and hourses of HorseStartersKits but want to replace the rider’s character with some other character from a different project. How can I do so?
Basically i want to replace the basic mannequin rider and use some other third person character in its place, the new character that i want to use is from some other project .

It sounds like you may need to do some retargeting. I have found a helpful tutorial you can follow and watch if you feel inclined to do so.