FPS Game Starter Kit uses the default Epic Games skeleton and meshes for the character. Any retargeting tutorial out there still applies.
Right click on your mesh and go to Skeleton > Assign Skeleton
Find the UE5 skeleton and click Accept
That’s it!
Open BP_CharacterBase or Replicated_Character and swap out the meshes with the new one.
If the head or other parts are causing you trouble, and the mesh doesn’t have material slots for body and head, etc., you can follow the Blender tutorial from here how to fix the mesh.
If your mesh is using the UE4 skeleton, same procedure, find the mesh, right click, skeleton, assign skeleton.
Find the UE4 skeleton and click Accept
Except it has an extra step, which is open BP_GameMode and set BPC_Character_UE4 as the default pawn class
Open the UE4 character child class and change its meshses to the ones that have the UE4 skeleton
Old video example of how to do it (still check out, just select the correct skeleton):
For more info about retargeting I recommend checking out these links:
FPS Game Starter Kit 