reset uNetbootin USB key from Read Only to write-able

I have tried to use uNetbootin to set up a live USB key. It has however locked into what may be a 'CD Install' mode as it only offers option to install. The key is now "Read Only" if you try to overwrite it in uNetbootin.
So it will not now overwrite with a Live Linux , even from the listed distributions in the download list.
How do I reset USB key to say writable, so it can be reformatted and retried. Thanks in advance.