Thanks for your posting! You can find the upgrade documentation here:
You would basically need to copy the TestRail files from the new version into your existing TestRail installation directory (thus replacing all files and adding some new ones). When you then access TestRail with your browser and TestRail detects that a database update is necessary (which it is from TR 4 to 5), TestRail would automatically show an upgrade wizard to complete the update. It’s basically a two step wizard that automatically detects your current TestRail database version and then applies all updates automatically.
We always recommend making a fresh backup before updating to a new TestRail version just in case but the upgrade will also mention this.
There’s also a command line version of the updater which is useful to automate the update or for large TestRail installations/databases:
I hope this helps and please let me know in case anything is unclear.