what button is for rotate building blueprints?

[Cop3]

?

[Chronic213]

It's scrolling the mousewheel, but it doesn't work sometimes. If that's the case, you can try placing the blueprint close to where you want it, then build it, then open your inventory while looking at it. You can then click and drag and use the mousewheel while holding leftclick to rotate or move it afterwards. I think this is a bug or not intended, so it might get removed soon. Hopefully not.

[Faustus]

Another way: Scip the Blueprint and put a Item from you Inventory on the Ground.
Rotate it, put it back into your inventory and go back to Blueprint. Rotating should work now.