Since Updates are sent via email we treat them as such, thus you are not able to recall or edit a sent Update.
If you wish to make a change and re-send an Update that was already sent, you can do so from two places.
1) Start next update
If it is the most recent Update you have sent, you can click the "Start next update" button and this will duplicate your most recently sent Update and allow you to make any changes.
2) Duplicate from Sent Updates
If the Update is older, you can go to "Sent Updates" from your sidebar, click the ellipses and then select Duplicate. This will duplicate the respective Update and allow you to make any changes.