Instructions
Visit a Pokemon healing center. The centers are indicated by an orange roof and are located in every city.
Walk to the computer to the right of the room. Click on the PC and select "Someone's PC." Highlight and click "Move Pokemon."
Cycle through your Pokemon and click the Pokemon you wish to release. You may have to flip through several pages of Pokemon before you find the correct one.
Click "Release." The game will inform you that once the Pokemon is released, the action cannot be undone. Click "OK" to release the Pokemon.