Silly mistake changing the permalinks in settings/ general

Hi
I have made a silly mistake. I am writing a site using WordPress on host 123 to replace a site still running elsewhere. I want the new site to have the same web address as the old one (www.fizzics.org) but it is not yet ready. Without thinking I went to Settings / general to edit the permalinks and replaced the temporary number code 46.32.240.33 in the web address with the fizzics.org address, knowing it was a step I would have to take but not reasoning that it was silly to do it before I was finished. Now when I try to log in to my new site I am simply diverted to a “catch all” page on my old site and therefore cannot get in or make changes or reverse the mistake. Is there a way around this without redirecting my url which would disrupt the flow to my existing site at a busy time of the year. I would appreciate your help. Thank you.
Roger