Scroll down to General App Information in the submission form.
Hover your mouse over the unwanted build version reveals a a small red Delete-like button to the RHS on same line.
Click the red Delete-like button.
The line that contained the unwanted build version is replaced with "Upload your builds using one of several tools. See Upload Tools / Select a build before you submit your app. [button]"
Click the newly revealed button to choose the intended binary via dialog, the same picker dialog you saw first time you chose binary for the submission form.