I have the same question but none of the answers above help. I currently have 5 builds for the same app version. 1 was expired and I expired another 3.
The help.apple link above sounds promising but I already saw that. The 5 builds I have are on the TestFlight tab, not the App Store tab. If the information in that link is correct it is very confusing because it looks like you are deleting the app, not individual builds of the app. I don't want to delete the app.
The YouTube link above is for deleting an app, again, not what the original question is asking.
So how do you delete an old/expired build?