You want to update the status of issues in JIRA using CSV? In this case you would use Bulk Move: https://confluence.atlassian.com/display/JIRA/Modifying+Multiple+('Bulk')+Issues
If you need to update a field in CSV file, you can open it using Excel and update it, before importing in JIRA, or when importing you can map the field and set it to In Progress when mapping the field. You can check more information here: https://confluence.atlassian.com/display/JIRA/Importing+Data+from+CSV
Hope it helps,
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