You could switch it out by clicking on it then clicking switch and clicking on another Pokemon. Or you could put it in the PC by going to the PC in the Pokemon center and hiting deposit Pokemon. Once it's in the PC you can relese it if you really want.
you cant
no cheat for platinum
you cant delete your Pokemon from your party directly but you can deposit them into the PC at the Pokemon center or you can release them if you wish to get rid of them the option is in your PC i do believe.
You don't "sell" Pokemon. You go to your party or PC box, click the Pokemon you want to get rid of, and select "release".
Go to a pokemon center and use the computer. deposit pokemon you want to get rid of and withdraw the ones you want to replace it with.
put one of your strong Pokemon in the front of your party then use (a) repel(s) (this should get rid of all Pokemon temporarily). or hack.
you dont need an ar just release them
Go to the Pokemon Center and go to the PC and select the Pokemon that you want to get rid of, and choose release. If the Pokemon is in your party, move it into the box of the PC, and then again choose release. :)
Your Pokemon Party are the six Pokemon which you take with you
What do you mean 'how do you party pokemon'
in any pokeCenter (the place where you heal your Pokemon) there is a PC (in the GBA games its on the desk) go to it and click A and you can move Pokemon between it and your party
This is how you get party Pokemon you capture Pokemon.