Change the mesh on a skeleton ?

Hi, maybe my title isn’t correct but I have no idea of how to explain it correctly.

I’m using the Pistol Animset Pro and the GR male hero 02.
And I’m trying to use the character with the Pistol Animset pro but I can’t figure things out.

For me animations are still black magic, but I’d like to learn how personna works.
I’m using the 4.13 version of the engine

Thank you.