Changing your bet is a great way to shake the machines up a bit, and can increase your playtime. On the bottom left of the game screen you will see your "Total Bet" with a "+ bet" and "- bet" buttons underneath. You can adjust your bet using these buttons, remember that the bigger your bets the bigger your wins!
Please note: The player will have a limit on the bet limit based on the level. Some slot machines containing special features (such as a Progressive Jackpot) which require a minimum bet to be eligible for that feature (i.e to win that game's jackpot). Please check each game's paytable for details on these minimum bets.