MikeJones do we have to update core extensions one by one too?
Running that command should update all extensions that can be updated.
Arnold But since no extension working with beta 13 is compatible with beta 14, should we uninstall them all?
If an extension has not yet released a beta 14 compatible version you'll need to uninstall it before updating. If there are extensions you need that have not yet been updated, we recommend that you wait for that.
Arnold Or if an extension has beta 14 update ready it can be disabled and then updated and enabled after flarum core update?
That is correct.
One thing to keep in mind is that if you installed a specific version of an extension, you might need to remove that from your composer.json, or composer might not want to update flarum. You'd see an error message about that though.