I can’t really give you a tutorial but i can tell you how i made it. Go to UE4 and export the Skeletal Mesh you want to reskin. Go to your 3D Software (i use Blender). Import the Skeletal Mesh. Now you should have the Mesh and the Skeleton or at least part of the Skeleton in your 3D Program. Select your Mesh and look for Vertex Groups, you should see at least one bone there. Now you have to look for the weights the Mesh has on these Bones. Import your own Skeleton (Mixamo) and be sure the Bone Names are matching the UE4 Skeleton. And then assign the Mesh to your imported Skeleton and copy the weights.
I know for a Beginner this could be a bit confusing. Heres one Blender Tutorial where i learned the Basics of Weight Paints and Skinning: https://www.youtube.com/watch?v=EI0bujHVMPM
yes, very confusing, over the last few day’s when I have free time I have been researching this topic since my knowledge == null, I have been watching these tutorials which will hopefully shed some light on the topic