Recalling the participants list before validation
When a Correspondent clicks Send participants, the buttons to add participants disappear, but as Correspondent you can still click Recall participant list to edit the list further. As long as the organisers didn't validate the delegation you can recall the list and immediately continue to edit it:

When you are finished, click Send participants and Confirm:

If by chance you and the organisers changed something in the delegation at the same time, you will be asked to refresh your page:

Recalling the participants list after validation
If you want to update the participants list after the organisers validated it, click the button Request recall participant list. This function is available up to and including the day before the meeting.

Enter a justification text in the text field of the pop-up window and click Confirm:

Then wait for the organiser's feedback. AGM will notify you once the organisers approve or decline your request. If approved, you can change the participant list and then send the list again to the organisers through AGM.