Insert DVD on machine
Backup /etc/zypp/repos.d
Remove /etc/zypp/repos.d
Execute Yast → Software → Software Repositories → Add → Use DVD option
Select the repositories that you need
Copy SLE-15*SP*-15.*.repo file to the new /etc/zypp/repos.d
Replace SPx and 15.x with the correct version… for example SP4 to SP5 and 15.4 to 15.5
Run zypper refresh -s -f
Finally run zypper dup -D –no-allow-vendor-change –no-recommends #that will test everything is OK
Perform zypper dup –no-allow-vendor-change –no-recommends to complete the upgrade
Reboot the vm
Tadaaaaa!