12.1.4.2  The upgrade procedure:

https://doc.opensuse.org/documentation/leap/startup/html/book-startup/cha-update-osuse.html#sec-update-zypper-proc


The various zypper commands refer to 15.3, but should be 15.4.  (Otherwise
nothing will get upgraded.)  I'm not sure if the first few steps should also be
15.4 (I think they should?), but definitely
  sudo zypper --releasever=15.3 ref
and the following 'zypper dup' should be using 15.4.

Additionally, the example given of removing and re-adding a repo under a new
URL should encourage modifying the repo file to use $releasever rather than
hardcoding a version number.