Start the game (log in), press the alteil button and then press the file button. You're now in the file editor (or deck editor as it'd be named in other card games). You'll see ten bars with the numbers from 1 up to 10 on them. These are file slots where you can create a new file. Your file should be in the first slot so if you want to change it press the button with 1. Now press the ok button, you can find it at the bottem of the screen.
The cards in your library should appear to the left of our file. Just press a card in the library and then press a card in your file to switch them. Below the library you see the buttons back and next which help you cycle trough you're library as one page usually doesn't show all your cards. I hope that helps, but feel free to ask if there are still questions.
On a side note you should be able to beat the level 5 NPC with a non edited starter. You can always ask people for advice on this. It may be useful to read these threads first though, they're about basic game mechanics and the current starters:viewtopic.php?f=13&t=6892viewtopic.php?f=13&t=13700