Step 1: Open up Your project
Step 2: Crop your image
Click on the "Toolbox - Tool Options" window and click the Rectangle Select, Then Select the part of your photo you want cropped, then click on Image>Crop to Selection and it will remove the part that is not selected.
Step 3: Change the Brightness
Click on Colors>Brightness-Contrast. The brightness slider makes it Brighter/Darker and the Contrast slider makes the colors show more if you move it towards the direction of the brightness slider.
Step 4: Export your files
NOTE: You don't save your file because if you do it will say that you can only use "save" for saving it in a GIMP File Format.
Step 5: Compare the difference
There is a large difference.