Upgrading? You should be reading the upgrade guide! (But then you wouldn't be here, right?)
Check that you can operate your system without any additional packages. The binary format changed, so you'll need to strip away all packages:
Before you remove anything, check package notes for any manual operations you need to perform. Then (if you care to), record what you had installed:
pkg_info -mq >/root/pkg_list_manual pkg_info -q >/root/pkg_list_full
Remove all packages (but keep any firmware packages):
pkg_delete -X /var/db/pkg/*-firmware-[0-9]*
Also if you installed anything that did not come via the package system, uninstall that too. Any local builds need to be rebuilt.
Get sets (bsd.rd at a minimum) -- the 5.5 sets include install55.fs and miniroot55.fs, USB install images, full install sets and bsd.rd only respectively. Boot the bsd.rd, choose U)pgrade.