Make sure you're logged into the right account and follow these steps: https://goo.gl/ruFcXC . If eligible, you can claim a new URL right after, but it takes a few days for the old one to stop directing to your channel. Also, you can remove a custom URL up to 3 times per year!
Thanks for reaching out. In the "Get a custom URL" box, you'll see the custom URL(s) you've been approved for. You can't change the part in the gray box, but you may need to add a few letters or numbers to make the URL unique to you. Hope this clarifies.