You can edit the Collection which will create a new version, privately. Changes can be made in two ways, either by adding new items or updating the version of existing items in the Collection. To make these changes publicly visible you'll have to click 'Publish' again, which is prompted in the interface after you make the changes.
If you have any further questions about the above, please let me know.
I'll also update our support pages as I appreciate that this isn't very clear.
If you need help or a step-by-step guide on how to edit your Collection, please also let me know as I'll be happy to help.
Operations Manager @figshare
Thank you for you reply. This is very helpful. When I read that the collection was "permanent" once published, I assumed you could not make any additions. Thanks for answering.