Once you have a pixel selection in place, it can be modified in several ways.
You can modify your pixel selection by enlarging or shrinking it, applying a feather to its edge, and/or by smoothing its curve. These options are available from the Select menu as Grow/Shrink, Feather, and Smooth, respectively.
In addition to the options mentioned above, you can move or transform a selection using the Move Tool or edit it using pixel-editing tools in Quick Mask mode.
The following settings can be adjusted from the relevant dialogs: