The following procedure will help you to unmount and detach a VMFS datastore:

1. Connect to vCenter Server using the HTML5 client and navigate to the Storage inventory.
2. Right-click on the desired datastore and select Unmount Datastore...:

3. In the Unmount Datastore window, select the ESXi host to unmount the datastore from and click OK:

4. The Recent Tasks pane will show the successful completion of Unmount VMFS volume:

5. Once done, go to the Configure | Storage Devices tab of the ESXi host the datastore was unmounted from, select the device corresponding to the VMFS datastore, and then click Detach:

6. On the Detach Device window, click YES:

7. The Recent Tasks pane will show that the Detach SCSI LUN task has completed successfully:

8. Once done, the storage device should be listed as Detached:

This completes the process of unmounting a datastore and detaching the storage device from the ESXi. You can now safely unmap the LUN from the ESXi host by using the array's storage management system.