Given the drive monitoring under the NetXMS Windows template is generated based on instances, here are two ways I know of.

Better option:
To add override capability to that template - this thread explains how to do that.
https://www.netxms.org/forum/configuration/dci-template-manual-override/

Not so good option: You may have to do this for all your drives as the template item to disable may also need to be the instance version.
If you want to disable the template and simply add your own - the following thread explains the disable part. Duplicate the template version first - update the duplicated one - then disable the template one.
https://www.netxms.org/forum/configuration/changing-template-reapply/

As Victor explained - the warning is correct - you are trying to update the template version - which can, and will, get overridden.

What you need - Duplicate, update the duplicate, and disable the template one - try that first. Below is how I would do it - or at least try it.

1. Find DCI item you want to change.
2. Right click - Duplicate
3. Right click - Disable (Goes yellow) - Template version now disabled.
4. Right click on the Duplicate (At bottom of the list of DCI's) - Edit
5. Make changes in Thresholds as per what you want
6. Click Apply and then OK

All done

This should work on the basis disabled template items stay disabled even after e template update.