When you are uploading a new CSV to a project or directly into your Research Hub panel, you can either map the columns to existing ones or create new ones. Existing data will not be updated for existing records unless there is an email address match and the columns have been mapped.
For example, participant Joe (firstname.lastname@example.org) is added in your original upload with "Project Manager" as his title in a specific column. If you upload a new CSV, map the job title column to the existing one, and change the title in your file to "Senior Project Manager", Joe's title will be updated. If columns are not matched, it will simply create a new one.