As with everything else in photoshop there are 200 ways of acheiving the same effect.
You can use a circular selection and then copy/paste the blue and or green information in the channels pallett thats one way, but its complicated and does nothing special really.
You could just paint out the red with black as suggested or use the color replacement tool (I hate the American spelling of colour, but thats what it is in PS)
Just make sure the foreground colour is what you want the tool to replace the red with. Use a small soft brush and make sure there is a tick in the anti-aliased box on the tool bar or you could end up with jagged edges.
Oh and dont forget the golden rule with everthing you do in Photoshop:
Never ever work on your original images, make a copy and work on that.

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