Hi, you can change it. If you are an admin of their account you can change it, if not they have to change it.
click on Account Settings in the top menu bar >> then on Company Settings. Scroll down the page, the last line is your login url.
Click Save after changing.
Hope this helps
It can be changed, but the old name won’t stay as a redirect so it’ll invalidate the links in any existing invoice or statement emails you’ve sent out in the past.