Mastering options are available only to subscription users, but accessing the feature is straightforward.

First upload a file, and on the preview page click the "mastering options," button below your audio file preview:
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Once clicked the mastering options menu will appear. Here you can click on any of the specific mastering options to adjust each parameter.

Once you're ready to hear your changes click the "Remaster," button on the bottom left of the menu. This will allow the engine to re-master the audio file with your new changes applied:
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Please note that clicking Remaster will override your current mastered version.