How do I apply a texture to multiple stairs at once?

Development Rendering
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2

I guess you mean stairs created with the respective brush inside UE4.

This goes for all the brushes (not just stairs):

  1. Select a face i.e. any “side” of any stair. Be sure to select a face and NOT the brush.
  2. Hit “ctrl + b” to select all the faces.
  3. Drag 'n drop the material from the content browser.