The rules governing the use of a sideboard differ for a limited (booster draft) tourney and a constructed tourney.
In a constructed tourney your sideboard must contain exactly 15 cards at all times during play. Therefore, you must exchange cards from your deck with cards in your sideboard on a one to one basis. In a limited tourney there are no restrictions on the number of cards that can be exchanged from your sideboard so long as your deck contains a minimum of 40 cards. In a limited tourney you may add or remove any number of basic lands so long as your deck contains a minimum of 40 cards.
The deck and sideboard must each be returned to their original compositions before the first game of each match.
