Updated Feb 2012.
First, you need to open your image and then duplicate it in the layers panel by right-clicking and selecting 'Duplicate Layer' or you can go the Layer's menu and select it from there. This is something you should always do when editing photos so if something major goes wrong, you always have the original to go back to. Feel free to rename each layer too if you so wish. You do this by double-clicking on the layer, typing the new name in the text box and hitting the enter key when you're ready to apply the changes.
Next, go over to the Tools and select the Zoom Tool. This will let you zoom into the area you’re working on and make it easier for you to apply changes to the image (you can use the + and - keys on your keyboard to adjust the zoom).
Make Your Selection
As the eyes are what we want to brighten, select the Free Select Tool so we can make a selection around them.
Towards the end of the Toolbox you’ll see a series of squares under the Free Select title. Make sure the second one a long (Add To Current Selection) is selected so when you move to select the opposite eye, the selection for the first isn't removed. Having antialiasing ticked smooths the selection. You can also tick Feather edges to further soften the transition between the selection and none-selection parts of the image but there's no need to do this for this tutorial.
You then just have to click and draw around each iris until you have two circles of 'running ants'.
Go to the Colours menu and click on the Brightness/Contrast tool. This will open a new window where you can adjust the brightness and contrast by moving sliders or by inputting a figure into the box to the side of each control.
Starting with the Contrast, adjust the sliders accordingly (the preview will show you how they’ll change so make sure the Preview box is ticked), move on to adjusting the Brightness then when you’re happy with how the eyes look, click OK.
Next we want to adjust the sharpness of the eyes and we’re going to use a Unsharp Mask to do this. Go to Filters>Enhance>Unsharp Mask to open the appropriate window.
Again, you’ll see a series of sliders (Radius, Amount, Threshold) which when moved, will adjust the image. Once you’re happy with the look, click OK for the changes to occur.
Once complete, zoom back out and see how changing the eyes slightly can help enhance an image.