First of all, don’t do how I do. Upgrade your installs regularly. OpenBSD makes it very easy.
But, if you do happen to get behind…
sysupgrade is very likely to fail.
Lots of 404 errors.
sysupgrade utility tries to grab the next version of the OS from one of the many mirrors
(the specific one your system will use is in
The default mirrors only keep the last 2 or 3 versions around,
sysupgrade constructs the url and tries to hit it for downloads, it will fail.
There are a couple of mirrors that keep almost all the old versions around.
https://mirror.yandex.ru/pub/OpenBSD/ has files going back to OpenBSD 2.x - they seem like the most serious archivists, at least of the mirrors I looked at.
https://mirror.sjtu.edu.cn/OpenBSD/ has files going back to 6.5 as of this writing (2022-11-11), also not too shabby.
Do a little
vi /etc/installurl and change the link to one of the above,
depending on how delinquent you’ve been.
That should allow you to do serial
sysupgrade commands until you catch up.
When you get close to the current version, consider switching back to a closer mirror, both for faster installs and to be kind to the folks who just saved your bacon.