Once at the Mastering Options page click on the "Volume," section of the Mastering Options Menu:
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From here you'll be shown the main Volume Menu.

You can then raise the fader to change the volume of your file to the proper setting you'd like to test:
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Once you're ready to hear your changes click the "Remaster," button on the bottom right of the Mastering Options menu to re-master the file with your new Volume change:

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