Tested on OpenBSD 6.4

Upgrade OpenBSD on Vultr

If you have OpenBSD installed on Vultr and want to upgrade it, then backup your data first.

Prepare

# ftp -V https://cdn.openbsd.org/pub/OpenBSD/6.4/amd64/bsd.rd
bsd.rd       100% |*******************************|  9782 KB    00:00
# ftp -V https://cdn.openbsd.org/pub/OpenBSD/6.4/amd64/SHA256.sig
SHA256.sig   100% |*******************************|  2141       00:00
# signify -C -p /etc/signify/openbsd-64-base.pub -x SHA256.sig bsd.rd
Signature Verified
bsd.rd: OK
# cp /bsd.rd /bsd.rd.63
# cp bsd.rd /
#

Read the official FAQ.

Update your configuration files, if needed for 6.4.

Access the console

Login to my.vultr.com.
Navigate to Servers > Instances and select a server.
Click on View Console icon.
In a pop-up window login with your password.

Upgrade

OpenBSD/amd64 (xxx.xxx.xx) (tty00)

login: root
password: ********

OpenBSD 6.3 (GENERIC) #100: Sat Mar 24 14:17:45 MDT 2018
...
# reboot
...
boot> boot /bsd.rd

booting hd0a:/bsd.rd: 3511114+1500160+3892040+0+598016
...

Choose (U)pgrade:

Welcome to the OpenBSD/amd64 6.4 installation program.
(I)nstall, (U)pgrade, (A)utoinstall or (S)hell? u
...

Answer the questions:

Which disk is the root disk = sd0
Force checking of clean non-root filesystems = no
Location of sets = http
HTTP proxy URL = none
HTTP Server = cdn.openbsd.org
Server directory = pub/OpenBSD/6.4/amd64
Set name(s) = done
Location of sets = done

Wait for the installer and boot normally:

...
CONGRATULATIONS! Your OpenBSD upgrade has been successfully completed!
...
boot> boot /bsd
...

OpenBSD/amd64 (xx.xxx.xx) (tty00)

login:

Close the console.

Update packages

# pkg_add -uv
...
#