Login to the website and find your game or training ( either in Schedule or list under the team ). Open the details of the game / training and you will see a blue bin on the right hand side of each game. This is the delete button, and when you click that you are prompted whether you want to notify the team or not.
On the mobile app - find the game, open it up so that you get the game view. On the About tab, you'll have a pencil icon towards the right. This indicates 'edit' - hit this. Scroll to bottom and you'll find a "Delete Game" button. When you do this, you also have choice of sending out messages ( push or text ) to your team to let them know.