So, you're in the Designer (you either went through a Client then clicked Design, or you're in a built-in sales meeting workflow). All you need to do is click the rooms tab

To apply a room, click on the slide that you'd like to apply. If this is a stock room, you'll see an Apply button only:

Or if it's a room that you've uploaded (and already calibrated):

Click "Apply" and the Designer will update with the room and resize the products on the wall to match the room's calibration.

Just like this:

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