Load the tournament, change info, save the tournament.
If you want to change everyone's buy-in, change it on the Game tab. Once you change it, the TD will ask you if you want to change the buy-in for everyone who has already bought-in.
If you want to change a single player's buy-in, change it on the Players tab. Double-click a player, then on the Player tab, double click the player's buy-in. Change the amounts. Press OK. Press OK.