Quick fix (Blender)

1. Select your mesh.
2. Go to Edit Mode.
3. Press A to select all faces.
4. Press Shift+N (Recalculate Outside).

If only some faces are wrong

• Select the wrong faces.
• Use Alt+N → Flip.

How to check

Enable Viewport Overlays → Face Orientation:

• Blue = outside
• Red = inside

Roblox note

Roblox uses backface culling on many materials. If your normals are flipped, faces may look invisible from one side.