If you read the general instructions
here, this should get your planning started.
Note: upgrades do not automatically make changes to /etc and /var, these must be done manually. A review of the following -current FAQ, which lists all of the architectural changes to-date since 4.4-release, will let you know what needs doing. And .... the sysmerge(8) tool mentioned in that announcement will automate anything not covered by the binary upgrade. In general. You'll still want to inspect that FAQ just in case.