You need to be aware that there are two components to upgrading a Rails app.
The first and most important part should be done by a seasoned developer. This happens on their local dev environment. The developer changes Ruby, Rails and dependent gems and may need to rewrite parts of your app to ensure it still works. Some gems that your app uses may no longer be maintained, and your Rails code may be using code that has been removed in newer versions. This effort required here is depends on the age between your current and destination versions.
The second piece is upgrading your hosting environment. Some hosting platforms make this part super easy, but it’s generally not too difficult to accomplish.
If you do 2) before 1) you will most likely have a completely broken app. That’s why there is no automatic upgrade path.
Bottom line: you need an experienced developer to upgrade your Rails app.