This step differs slightly from data import. You must first create a match field. It'll be used to identify existing records in your app that need to be updated. Please keep in mind that this value should be in one of your CSV's columns. And you need to map this when setting up mappings. It will appear under Match Field in the dropdown menu.
If you need to replace your CSV file, click Change File and upload a new one.
Set up mappings. Please read an article on types mappings
if you have questions.
When you're done, click Start Import. Please keep in mind that updates may take some time. It is safe to close your browser now and return later.