Short answer: You can't (unfortunately, and at the moment).
When you enter the base URL during DAVx⁵ login, DAVx⁵ retrieves related information from your server, including the home-set URLs, the actual calendar URLs, who they belong to (represented as an URL) and much more.
Now when your CalDAV URL changes on the server, it means that all that information is not valid anymore. So everything has to be detected again, which means the whole setup process would have to run again. For DAVx⁵, the existing calendars didn't just change the URL, but "everything" changed. Imagine that the URL changed because the server software changed – it means that URLs might have a different structure and even behave differently.
Of course, DAVx⁵ could just delete the old calendars and run the whole setup process automatically again as soon as you enter a new base URL. However, at the moment this is not possible. So the current solution is to delete the old account with the old base URL and then create a new account with the new URL.
Attention: In the case that DAVx⁵ can't synchronize because your old URL isn't accessible anymore, there may be unsynchronized local changes (for instance, a new event). These changes will be deleted when you delete the old account.
Currently there is no good solution for that problem. If you can control the URL change yourself, make sure that all changes have been synchronized before changing the URL. If you have no control about the URL change, this usually means that you use a professional hosting provider, who can use redirects or provide a migration phase where both URLs are working. In this migration phase, sync your changes with the old URL, then delete the DAVx⁵ account on your Android device and create a new account with the new URL.
We know that this is not a good solution, but this is a really complicated matter. (By the way, other clients like Thunderbird or Evolution don't let you change the URL even for a single calendar, too.)
All said above applies to CardDAV, too.