I show this using a certain mirror. One can select what ever mirror they wish from

http://mirror.openmandriva.org/README.txt?mirrorlist

just be sure you pay attention to package verisons.

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To upgrade an OM Lx 4.2 x86_64 system to the Cooker systemd-248 package stack. For other arches just navigate back up the tree on your mirror to find your arch.

First for backup I download the current OM Lx 4.2 systemd stack and put all of then in their own folder with no other .rpms. 

To get them go to:

http://omlx.northrepo.ca/4.2/repository/x86_64/main/release/

There 15 packages:

lib64nss_myhostname2-247.20210207-1
lib64nss_resolve2-247.20210207-1
lib64nss_systemd2-247.20210207-1
lib64systemd0-247.20210207-1
lib64udev1-247.20210207-1
systemd-247.20210207-1
systemd-analyze-247.20210207-1
systemd-console-247.20210207-1
systemd-coredump-247.20210207-1
systemd-cryptsetup-247.20210207-1
systemd-hwdb-247.20210207-1
systemd-locale-247.20210207-1
systemd-macros-247.20210207-1
systemd-polkit-247.20210207-1
systemd-resolved-247.20210207-1

These 14 packages are the current systemd stack in Cooker. To get them go to:

http://omlx.northrepo.ca/cooker/repository/x86_64/main/release/

pay attention to package versions as there are 3 different versions of the systemd stack in Cooker main/release repo. For this there are 14 packages one less than for the other verson. Don't worry, dnf will know what to do. (systemd-resolved-247.20210207-1 will be obsoleted). I put these in a folder with no other .rpms.
 
lib64nss_myhostname2-248.20210308-3
lib64nss_resolve2-248.20210308-3
lib64nss_systemd2-248.20210308-3
lib64systemd0-248.20210308-3
lib64udev1-248.20210308-3
systemd-248.20210308-3
systemd-analyze-248.20210308-3
systemd-console-248.20210308-3
systemd-coredump-248.20210308-3
systemd-cryptsetup-248.20210308-3
systemd-hwdb-248.20210308-3
systemd-locale-248.20210308-3
systemd-macros-248.20210308-3
systemd-polkit-248.20210308-3

If you install all those to a folder with no other .rpm's you can install these with:

$ sudo dnf upgrade *.rpm

And reboot.

If there is a problem you would navigate to the folder with the original systemd-247.20210207-1 packages and downgrade to those with:

$ sudo dnf downgrade *.rpm

And reboot.