By default we will only add data to blank fields during an upload, not delete existing data if the corresponding field in your upload file is blank. However, if you check the "Clear blank values" checkbox when uploading your file (if you have the "Advanced Upload" permission for the group), we will clear out any data on each matching activist that is blank in your file for columns you decide to match on step 2 of your upload.
So, for example, let's say an activist had a first name in the system, and you uploaded a file with their email address but no first name in the first name column. By default, we would keep the first name we have, even though you had a blank space in your file. If you check this box, we will delete the first name value instead, making it blank.
In this way, you can mass-delete values from fields on your activists. Be careful with this option, it cannot be undone.