You’ll need to follow the upgrade path for major versions explained in Upgrading GitLab | GitLab
Take a backup first (full VM snapshot recommended).
Issues to expect:
- Upgrading from 11.x to 12.x to 13.x to 14.x to 15.x requires a lot of upgrade steps and database migrations. Plan with downtime, depending on how big the environment is.
- Ubuntu 14 LTS is quite old, too. In between the GitLab major versions to upgrade, you may need to upgrade the Ubuntu distributions too, because newer GitLab packages will have dropped support for Ubuntu 14 LTS becoming end-of-life. Supported operating systems | GitLab