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In In your project, go to Data tab and select Import Cases from Excel.
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Select your file and click on >> Next Step.
On the next page, first choose your case type. If your desired case type is not listed, please create an application that contains that case type first (Case Managementhttps://dimagi.atlassian.net/wiki/x/ry7Kfw ). 
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Next step is to tell CommCare HQ in which Excel column the unique ID is and what type of unique ID you're using. Follow the table below
IF | THEN |
|---|---|
you're using case_id to update the cases (i.e. you did a case export from CommCareHQ), | choose |
caseid as your Excel column and Corresponding case field as Case ID (see first screenshot below) | |
you're using an existing Unique ID (ex. household number), | choose that column as your Excel column and Corresponding case field as External ID. (see second screenshot below) |
Existing Unique ID:
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No Unique ID: 
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Make sure you check "Create New Cases if there is no matching case"
Next: