Use Curves

Shortcut	Action
`Ctrl` `M`	Open the Curves dialog box
`+`	Select next point on the curve
`-`	Select the previous point on the curve
`Ctrl` `D`	Deselect a point
`Del`	Delete a point on the curve
`Arrows`	Move the selected point 1 pixel
`Shift` `Arrows`	Move the selected point 10 pixels

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