To change the selection edges, you can use the commands in the Select menu.

Modify Commands:

The Expand command allows you to move the selection border outward by the specified number of pixels.

Original Selection

Expand Command

The Contract command allows you to move the selection border inward by the specified number of pixels.

Original Selection

Contract Command

The Smooth command allows you to smooth the corners of a selection.

Original Selection

Smooth Command

The Border command allows you to select a line of pixels of a specified width along an existing selection border.

Original Selection

Border Command

The Feather command allows you to soften the edges of a selection by gradually decreasing the sharpness.

Original Selection

Feather Command

Grow Selection. The command allows you to extend the selection using adjacent pixels.

Original Selection

Grow Selection Command