Please find steps for the upgrade, this approach is a tested one.
Few things to be noted before embarking on the Upgrade.
The upgrade is a big leap from 7.x to 8.8.0 (latest version) There are a lot of changes between 7.x to 8.8 U can see the [Change Log](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG)
Multiple steps are need to upgrade to latest, direct upgrade is not possible.
Gitlab backup can be restored on the same version of Gitlab version. Eg: 7.1 backup will go in to 7.1 only, not on 7.2.
I recommend you take a backup before you embark on the upgrade from 7.x to 8.8 (latest)
Assuming you are on 7.1, direct upgrade from 7.1 to 8.8 or any 8.x might not be successful, earlier people have faced DB migration failing, to avoid this you need to take a few steps Please find them below
Gitlab 7.1 to Gitlab 7.4
Gitlab 7.4 to Gitlab 7.6
Gitlab 7.6 to Gitlab 8.0
Gitlab 8.0 to Gitlab 8.5 (optional , u can skip this step if you like and upgrade from 8.0 to 8.8)
Gitlab 8.5 to Gitlab 8.8
Please follow the upgrade steps
Note: You cannot restore your backup taken on 7.X on to the latest version i.e., 8.8
Run Rake tests in between every steps to check if the installation/upgrade was successful. This a cautionary measure.
sudo gitlab-rake gitlab:env:info --trace
sudo gitlab-rake gitlab:gitlab_shell:check --trace
sudo gitlab-rake gitlab:sidekiq:check --trace
sudo gitlab-rake gitlab:app:check --trace
sudo gitlab-rake db:migrate:status --trace
sudo gitlab-rake gitlab:check SANITIZE=true --trace
(try to solve all issues before progressing to next step of upgrade, U can ignore the hooks and other simple errors which come in the rake tests)
Configure your HTTPS/LDAP etc if needed.
I suggest you to try this on a test server and then do this upgrade on the actual server.
Please feel free to post any issue which you encounter during/post upgrade.