On the Roster page, simply tap/click on the numbers next to each player's name. You'll be presented with Up/Down arrows that will allow you to increase or decrease stats. If you use the Scorekeeper, stats will be automatically added when you add goals, assists and penalties.
Please note: If you have "Auto Games Played" or "Auto Goalie Stats" turned on, you cannot edit a player's games played manually or goalie stats. They will be calculated automatically.