I'm trying to fix this brighter part of a person forehead because of the over exposure. How can I fix this part to match the other color on the forehead in the picture?
This is a prime example of why it is wise to shoot in RAW. With RAW images, it is a simple matter to bring down the highlights in the exposure and spots like this will mostly disappear.
In the case of extreme highlights (even in RAW) or shooting JPEG, there isn't actually any information to replace the white spot with, so you will have to create your own. This is generally easiest using either the context aware heal tool or using the clone brush to manually build up skin texture over the white spot, though assembling a realistic look for that large of an area (including facial lines and such) is going to be extremely challenging.
This can be fixed with the patch tool in Photoshop, or simple cloning if the surrounding areas are similar. The healing brush is also a great tool, but depends more in the brightness of the surrounding pixels and the size of the brush your using to get it to work correctly. Posting a larger un-cropped image would help in this case.
Try to pull out as much detail from the raw as possible.
Duplicate the image in a new layer.
Fill in the area with the patch tool or healing brush or clone tool.
Adjust opacity to try to create a more natural appearance.
Use layer masks to hide any artifacts that may have been produced.
Here is a sample edit (original, clone, opacity):