If a meeting has been 'Locked' it cannot be edited until an admin unlocks it.
Go to Meetings
Find the Meeting you are after and select 'View Details'.
If you have access to unlock a meeting you'll be able to do so in the arrow drop down menu on the right.
Select Yes, Unlock Meeting.
Now you can Edit Minutes - for topics, tasks and decisions or Edit Meeting - for name, time, date and location info.