To edit a symposium you have already created, log into https://app.oxfordabstracts.com and click on the pencil icon, then Amend symposium on your dashboard. You can edit it, even if it is complete, subject to deadlines.
You can then amend any of the fields...
Make the changes required then click on Submit.
After pressing Submit, you then have to option to add, re-order (by dragging and dropping), or remove abstract submissions.