Replace file for response with File upload question

We need to change one of the files that was uploaded in a survey. The respondent contacted us with a replacement because there had been some mix-up.
We cannot find any way to achieve this. The edit Response feature leaves all file uploads greyed out. The option to Retake the response does not help either, because this also only shows the greyed-out name of the originally uploaded file, and does not offer any way to access or delete or replace this previously submitted file.
Does anyone know of a way to achieve this? It seems obvious to us that this should be possible, but we cannot find it, and the website does not seem to explain this either. (There is a tip: "Attention: Once a file is uploaded, it cannot be removed by a participant. However, the participant will have the option to replace the file with a different file.", but this seems to be possible only until the response has been submitted)
Any help greatly appreciated!