After rebooting into the upgraded system, run sysmerge. sysmerge with no arguments runs well if you have up to date source code around, otherwise feed it the etc and xetc sets, ie cd to where the sets are, run
sudo sysmerge -s etc55.tgz -x xetc55.tgz
If you get the warning about unknown keys, tell it to ignore this once. You may want to remove the files mentioned in http://www.openbsd.org/faq/upgrade55.html#RmFiles
Once you've rebooted, you can start reinstalling your packages:
pkg_add -z -l /root/pkg_list_manual pkg_add -za -l /root/pkg_list_full
But simply pkg_add pkgname will work too.