Instructions
Visit the Pokemon Center in your city. Phenac City, Pyrite Town, Agate Village, Mt. Battle, and Realgam Tower all feature Pokemon Centers. The Centers provide healing for your Pokemon, as well as access to your other captured Pokemon. Enter the building through the automatic sliding doors.
Access the PC. The PC stores all of the Pokemon you've captured. Upon entering the Center, locate the PC, which will typically be on the left or right side of the first floor. Press "A" in front of the PC to turn it on.
Select the "Pokemon" option on the PC. The left side of the screen displays the Pokemon in your party, while the right side shows the Pokemon in the computer.
Switch the Pokemon that are in your party. Select a Pokemon from the left side of the screen. Press on an open space on the right side of the screen to drop a Pokemon from your party into the PC. Choose a Pokemon from the right side of the screen and click on an open slot on the left side of the screen. This process switches the Pokemon in your party.
Repeat Step 4 until you've changed your active Pokemon party to your liking.