Upgrade Help from 12.4 to 15

Good Day.

I hope some people might be able to help me. I have been given the job to update our On-Prem server, We are running Ubuntu 18.0.4 and GitLab 12.4. I want to get this upto date, but I cannot find the correct upgrade path. Can anyone advise?
Also GitLab is connected to our AD and I can sign into Gitlab with no issues, but I do not know the Ubuntu Username either.

The recommended upgrade path can be found on