You should not have used ‘zypper update’ anyway, and ‘zypper’ tells you so. TW needs ‘zypper dup’. Now you missed the downgrade that was made available.
What you can do, is download the zypper and libzypp packages, install them manually.
As a new User to openSUSE,
You should know that unlike many other distros,
openSUSE differentiates between an
update - Used to install incremental improvements in features and security. Does not alter the distro version.
upgrade - Used to completely re-install the system from one distro version to another.
An update is used only for periodic releases like LEAP which releases a new version every 12-18 months, so between each release incremental improvements are installed as updates.
Since Tumbleweed as a rolling release distro releases a completely new distro version every few days or couple weeks at most, you are upgrading from one major release to another each time and there are no incremental updates.
This important tidbit and more are described in my Intro to openSUSE slide deck which describes pre-install planning, the install and first steps after installation for anyone new to openSUSE