Can you automatically/easily rename multiple folders and links?

My hosting domain is christianleonard.com. I got a new client, lets call them site123.com. I didnt have access to change their nameservers, so I set up a folder on my account, christianleonard.com/site123. I then created a wordpress site for them there.

Now, Ive pointed their domain to my server, and when you go there it shows the site I created. However, every page you go to has my domain in its url, ie, christianleonard.com/site123/gallery.

Is there an easy way to remove my url from their url? Or do I have to reinstall their wordpress site? Thank you for any info!