If you want to update data that already exists in one of your objects using an import, this article section will show you the steps: Updating an Existing Object.
There are two important items in that article section to pay attention to:
- To update existing records, you will need to identify a column in your file with unique values that help to identify which records should be updated in your object.
- If a matching record isn't found in your object, a new record will be created instead. This is a great shortcut for importing both new records and updating existing records using a single import.
When importing to update existing data, you do not need to include all the fields in your object within the import file. You can include only the fields you want to update in your import file, plus the field that you will use for matching (identifying which records to update).