Sorry about my german version of Photoshop.
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Select the Channel Window from the Top-Bar of Photoshop.
On top of the window is an icon of the RGB image. Below you see a red channel, a green channel and a blue channel.
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1. cmd A makes a selection of your image. See the running line.
2. Click on the red channel. This will activate it.
cmd C put the red channel into the clipboard. (you don't see any change yet.)
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3. Click on the green channel.
cmd V paste the red channel from the clipboard into the green channel.
Next and final step is to paste the red channel also into the blue channel.
If you click now on the RGB image icon the result will be a black and white image, chosen from the best channel.