Projects that use UE4 or are older then version 2.4 you’ll need to follow the guide linked below
https://defusestudios.com/survival-game-kit-v2-changing-character-mesh/
UE5 Mannequin Skeleton
If your new character uses the UE5 mannequin skeleton you shouldnt need to retarget, you can copy the video below to change the mesh.
Custom Skeleton
If your new character does not use the UE5 mannequin you will need to retarget, you can copy the video below to change the mesh.
Defuse Studios 