If you are running a version of the client less than version 1239, you need to remove it from your computer and manually download the new version.
We released the first version of this new client a couple days ago as 1237, but ran into a couple issues – please remove those versions and manually get this latest version.
Please let us know if you encounter any issues with this new version of the client by replying to this thread. We’ll investigate and resolve them as quickly as possible. It is a bit difficult for us to test on all potential platforms that people are running the client. Once we have hammered out any issues, this version of the client should be able to auto-update itself through the rest of BfA and all of Shadowlands.