At first is good to look support cycle
https://endoflife.date/opensuse
Good instructions for upgrade can be found from
https://en.opensuse.org/SDB:System_upgrade
Update the repos from Leap 15.2 to 15.3
sudo sed -i 's/15.2/${releasever}/g' /etc/zypp/repos.d/*.repo
su
zypper --releasever=15.3 refresh
zypper --releasever=15.3 dup --download-in-advance
When all packages are installed, run
reboot
After rebooting install updates vie update-manager
In Mate desktop seems to be a bug, which causes slow shutdown. To bypass that bug you can edit /etc/systemd/logind.conf by unticking line #KillOnlyUsers= as KillOnlyUsers=
