This isn’t all that easy to do and may be worth just retargeting. Otherwise you would need to use a 3d program like blender or maya to edit the skeleton.
There is an addon for blender the converts the Mixamo skeleton to a UE4 skeleton (i’m not sure though how well this works).
You could also re-rig the character mesh to the UE skeleton (you would need to export the skeleton out from UE first and then re-rig and re-import). Then use the animation converter to play the Mixamo animations on the UE4 skeleton.
(I think Mixamo used to have this option - but when Adobe purchased it they removed this functionality).