Select File>Import>GLTF 2.0 and then select the .glb file you downloaded.
Wait for the import to complete and the model to appear in your Blender scene.
Select File/Export/FBX. The Blender File View window opens.
Notice the Operator Presets on the right.
Adjust them as shown in FBX Export Settings below.
FBX Export Settings
These instructions assume that your current FBX export settings are the Blender Operator Defaults.
Path Mode to Copy for Embedded textures (Optional)
To include generated textures:
Set Path Mode to COPY .
Click the icon next to the Path Mode dropdown so that it changes and is highlighted blue. This enables texture embedding.
Set Apply Scalings to FBX Units Scale.
Uncheck Apply Units.
Geometry Tab (Optional)
These settings prevent import error warnings about smoothing.
Nothing actually breaks!
Set Smoothing to Edge.
Check Tangent Space.
Uncheck Add Leaf Bones.
Summary of Export Settings
2. Convert to Mixamo compatible skeleton
Importing the exported FBX straight from Blender into Unreal Engine and trying to use the RP_Mixamo_Skeleton will cause an error labeled "FAILED TO MERGE BONES".
Before you import your FBX file into Unreal Engine, you must upload the exported FBX to Mixamo and then download it again. This is because Mixamo applies modifications to the skeleton structure/transforms that make it compatible with the Mixamo animation library.
Without this step, there will be UE import errors that make the FBX file incompatible with the Ready Player Me reference skeleton and animation blueprint.