Do you want to replace the previously uploaded file? Then go through the same process as adding respondent and go to "Participants" and "Import". Upload the desired CSV file and select "Add" or "Replace".
As a check, the previously uploaded data is shown in the import process. This way you can compare the new data with the old one and make textual changes if necessary.
Note: you can only replace respondents with less than 20 completes. This is to avoid accidentally removing respondents who have already completed the survey.