Trying to switch to weebly, accidentally changed wordpress URL

I am coming in on the middle of a project for a client. They have previously set up a site here on wordpress, but I have set them up a new one on Weebly. Now I’m trying to switch their domain name to connect with my weebly site, instead of wordpress. I have their admin log in info, but I didn’t see anything about the domain name, so I tried to change the URL to point to weebly, apparently that was a mistake, I can no longer log in to their wordpress admin, and the site looks terrible. Please help!

Please keep in mind, I’m coming in on the middle of this project so I have limited knowledge and access (my clients have limited access as well, because they had someone else set it up thru word press for them)

Also, I have read all the suggestions about the PHP and FTP options but I have no clue about coding or anything web design really, so I really need a “for dummies” version!