When you receive inventory into a purchase order, it will automatically change the status from open to either partially received or closed. If you need to change it to another status follow these steps:
In the purchase order, click on Settings.
Then update your purchase order status.
Finally, click ‘Save Changes’ in the lower left corner.
There's an option to Approve a PO so it changes status from "DRAFT" to "OPEN". Select the DRAFT in Stock Orders, then select Bulk Actions and Approve Draft.