Are we talking about regular school spells or treasure cards? Either way, if you put cards in your deck you can take them out. Just like putting them in, all you have to do is click on the card in your deck and it removes it from your deck. You can do this with all cards in your deck and then go back and add whatever combination of cards you feel would be best in battle. This also is the same for treasure cards. You can add and subtract as many times as you want, just note you cannot do this in battle. Before going into battle, click on your book and then click on the deck icon on the right. Once in there, click on the left the cards you want removed and then on the right go through and pick the new cards you want to replace them with.
Is there any way to discard, sell, or just empty cards from a deck to make room for new ones?
>> You may remove a card from your deck (either learned spells or treasure cards <in sideboard>) by clicking on them. >> You may add a card to your deck (either the normal deck or treasure card sideboard) by using the pages on the right side, which display known spells (or in the treasure card section those you currently own) and add the cards to deck limits. Click on card to add. >> During play you may discard a known spell that has been selected for combat, using the discard button displayed on the screen. You may either leave empty, drawing more cards from your known spell deck the following round, or may use the draw button to randomly pull cards from your side deck (treasure cards) which can be used the round you draw them. >> Treasure cards can be sold (Bazaar).