Once you've accepted (or tentatively accepted) a meeting request, you'll have the ability to open and modify it in your own calendar — but you shouldn't. Remember, you don't own the meeting, so it's not really yours to change.
Opening the meeting to see its details is fine; however, it's bad practice to modify the details of a meeting that you don't own. Why? Because changes you make will be overwritten if the meeting organizer sends an updated request. That means that if you set a reminder for the meeting because the organizer didn't, your reminder will be overwritten if the meeting gets updated.