Before proceeding with this topic you need know how to work with selections.
This feature works only with RGB/Alpha Channel image formats.
Adjust brightness and contrast of a portion of image
1. Select a region with the selection tool. If no selection is active, the adjustments will apply to the whole image.
2. Choose Draw/Adjustments-Effects/Brightness-Contrast or press "B".
3. The dialog box Adjust Brightness - Contrast opens
4. To get a better idea of the adjustments you're doing, we recommend you to activate the Preview option. It permits you to see the changes in background (editor and preview windows).
5. Specifies the Brightness value you wish to add to the current image. The value you specify is added (or subtracted if this is a negative value) to the current image brightness. Valid values are in range -100 (dark) to +100 (very light).
6. Specifies the Contrast value you wish to add to the current image. The value you specify is added (or subtracted if this is a negative value) to the current image contrast. Valid values are in range -100 (low contrast) to +100 (high contrast).
7. When done, click OK.