Grow the current selection by right clicking on the selection and selecting the Expand Selection menu item. This will bring up a dialog asking how much to expand the selection by. Enter a positive number and click Expand.

Shrink the current selection by right clicking on the selection and selecting the Contract Selection menu item. This will bring up a dialog asking how much to contract the selection by. Enter a positive number and click Contract.

Feather or blur the selection boundary by right clicking on the selection and selecting the Feather Selection menu item. This will bring up a dialog asking how much to feather the selection by. Enter a positive number and click Feather.