I've uploaded my agenda already but there are still a lot of changes to make before the event. Do I have to do all of this editing manually?

No. If you have already uploaded your agenda but still have changes to make to your schedule, you can go to the "Agenda" section of the dashboard and click Import / Export > Export Agena Excel to export your currently uploaded agenda into an Excel spreadsheet template. You can then make edits directly to your excel spreadsheet, save it, and re-upload it to the dashboard to save your schedule changes.