As administrator, you should be able to visit that user's profile page (just click on their username) and edit it. However, I have just discovered that not all Nabble servers seem to have that feature installed. Keep watching my post at:
http://support.nabble.com/Can-t-Upload-File-tp7595912p7595939.htmland see if the reply (when it comes) helps.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.