There are two simple methods. First one is using the Curves (Shortcut: Ctrl+M). When the Curves dialog box appear select the Channel to Red. Then drag the curve slightly up dragging in the middle. While the Preview check box in checked you are able to see the image in change. So adjust the appropriate red limit and press <ENTER> to finish.
Second method is using the Levels (Shortcut: Ctrl+L). Set the Channel to Red. Drag the middle slider to left slightly. While checked the Preview checkbox you can see the changes on time. Press <ENTER> to finish.
