When adding an event (shot, foul, assist, etc) you can make mistakes. If you know immediately you made a mistake the quickest way is to delete and re-add the event (swipe to delete over the event at the bottom of the screen) If other events happened then navigate from the Collect Data page go to the event log (using the menu in the upper right). Once there locate the event you want to change and touch it. This will present edit options. Be sure to save after the edit.
If you are looking for an event that happened a while ago you can filter the list. Reduce the events to just the team you want with the buttons along the bottom. Then jump to the search field at the top (scroll up, or touch the magnifying glass icon in upper right, or touch the Time along the top of the screen to jump to the top). It will default to "Event" category. type in "FG" or "foul" to search by event and it will reduce the list to just the type you are interested in. Press the blue search button at the bottom of the screen to go to edit mode. Now touch the one you want to change and edit as before. You can change who the event went to or the value of the event. If you want to remove it completely then use a horizontal swipe to delete it.
You may need to hit "Cancel" on the search UI to see the back button when done.
You can also add events with the event log. Select the event log, then then "Edit" button in the upper right corner. Touch the green plus button next to the event you want to insert before. You will get a list of options for what event to add. You can also move events by dragging from right side handle when in edit mode but only within the same period.