Team Foundation Server 2015 includes a large number of changes to the schema used in its databases, and as such, an upgrade from TFS 2013 and older releases is expected to take a significant amount of time. Because upgrades are offline, Microsoft is providing a tool, TfsPreUpgrade.exe, which can be used to perform the most time-expensive portions of the upgrade online against TFS 2013 QU4 and QU5 deployments. The Upgrade wizard includes a readiness check, which warns you if your database is large enough that running TfsPreUpgrade.exe is recommended.
So lets start, my current TFS server has TFS 2013 Version 12.0.21005.1 and I am going to upgrade it to TFS 2015. First download TFS 2015 ISO and mount ISO on TFS server.
Before launching TFS 2015 setup, close TFS 2013 Team Foundation Server Administration Console.
Run the Visual Studio Team Foundation Server 2015 installation (tfs_server.exe) and click on Install.
Setup will take 5 to 10 minutes to complete the installation process and its depend on your system performance.
Upgrade Wizard opens automatically, at the end of the installation process. In our case we are doing an in-place upgrade,