Hello there, just want to give you a tip, Check the forum, and youtube they have very good tutorials.
To replace the model, you need:
1- The new 3D model with Topology for animation, UVS, All maps baked.
2- The model need to be Riged and with animations.
After tha process, you will have a Character model ready to use in any Game Engine.
Now for Unity you need to look for the workflow to import the 3D model with all the resources (textures, animations, etc). I dont know what is the workflow for unity, but i am pretty sure that you can find tutorials on youtube or any oficial forum of unity.
Just want to give you a little guide to help, nothing more.
Good luck!