I know that you can move the cards you have earned for your deck to your sideboard, but I was wondering if you can move original cards that you start with in your deck to your side board, or change the amount of mana in your deck.
I'll agree with what Dave said, and going by what is written on the official Wizard's site's FAQ:
Q: Can I create new decks or modify my deck?
A: Duels of the Planeswalkers allows players to customize their deck with cards unlocked by defeating opponents. The core of the deck is constant but how you choose to modify your deck is up to you.
And since the FAQ isn't split into platform-specific groups, the rules are the same: you're stuck with your core deck, and can only add/remove unlocked cards.
You cannot remove any of the original cards in your deck. You can only move unlocked cards to and from the sideboard.
The number of lands in your deck is proportional to the size of your deck: lands are added as you unlock cards or move them from your sideboard to your deck (and subtracted as you move unlocked cards to your sideboard). For example, you start with 24 lands in a deck of 60 cards. When you unlock your first card, no lands are added (24/61); unlocking the second adds another land to your deck (25/62). Removing one of the two unlocked cards also causes a land to be removed, so your deck is back to 24 lands in 61 cards.