Please note: To edit a calendar invitation, you will need to be the original sending user. This is a security privilege to avoid the scenario where another user may update a separate user's invitation.
To start, double click the invitation to edit.
From there, you may alter the 'attendee' list, the date/time, as well as add new notes to the meeting.
With that complete, you may 'send update' to then update the invitation with any new changes you have made.