How to update Gitlab
Gitlab is very picky about how updates must be installed. They use the following versioning scheme: (major).(minor).(patch). In order to update, you must first update to the latest minor version within a major version, then you can update major versions. More info here and here.
Before updating, stop gitlab: systemctl stop gitlab-runsvdir.service then make a snapshot on vSphere, in case something goes wrong.
Our installation is the gitlab-ee omnibus package. To update it, follow the version requirements from the above link, then click here and find the appropriate version of the package for Debian (including the correct version of Debian; at this time, stretch). Run the install command on the right side of the page.
After the update has been installed:
Prevent apt from trying to update gitlab: apt-mark hold gitlab-ee
Start the gitlab service: systemctl start gitlab-runsvdir.service
Reconfigure gitlab for the new version: gitlab-ctl reconfigure