That
OpenBSD flavor are you following?
If you're following -RELEASE, you need to wait for 4.9 to be released, if you're following -STABLE, you need to checkout the system source and rebuild, and if you're running -CURRENT (my favorite), you need to download a snapshot bsd.rd, boot from it, perform the upgrade, reboot, run sysmerge -s /path/to/etcXX.tgz (usually the same mirror you got the bsd.rd from, though that's not 100% critical), then pkg_add -u. Alternatively, you can download the source and build it for -CURRENT.