Use the Shape-Based Anti-alias command to anti-alias a selection based on its shape.
To anti-alias based on shape:
Choose Selections > Modify > Shape-Based Anti-alias to open the Shape-based Anti-alias dialog.
For the Anti-alias mark an option:
Inside to anti-alias the inside of the selection marquee.
Outside to anti-alias outside of the selection marquee.
Click OK to anti-alias the selection edges.
Adding or Removing a Color from a Selection
Changing the Feathering of a Selection
Cleaning up the Edges of a Selection
Modifying the Select Selection Borders