In every Pokemon Center, there is a blue P.C. that you can interact with. When you do, a list of options come up. CLicking "Move Pkmn" takes you to your PC Boxes, which displays all of the Pokemon you have at reserve. Going up twice and selecting "Party Pokemon" opens up a column with all of your party Pokemon, which you can select and move into your PC box for storage. You can also put Pokemon into your party by clicking on a Pokemon in your PC and placing it in your party column. I hope this helped