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Click the Edit icon (a square with a pencil) to enter Edit Mode. Click the dotted rectangle icon in the editing toolbar, then select "Rectangular Selection". Click and drag a part of the image you want to keep. Click Crop button. Press Cmd+S to save your file.

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Once you let go of the mouse button, you will see the dotted rectangle appear over a portion of the image. This will remove all parts of the image that are outside of the rectangular selection. You can resize the cropped area as you would any image. If you don't want to save your changes, press Cmd+Z to undo them. If you want to save the cropped area as a brand new file (and keep the entire image from which you cropped it), Click "File," then "Save As," then choose a new filename. To revert the image to a previous version after saving, click "File," then "Revert To," and select "Browse All Versions" Now, choose an older version of the image.