Work a double running stitch by going back and fill in the gaps of the running stitch.
Using buttonhole stitch, cover the double running stitch. Loosen the fabric a bit, and then start removing the thread (please see Drawn Thread Preparation).
Once you unpick the thread up to the buttonhole stitch, slide one blade of your scissors underneath.
Push the flat of the blade against the buttonhole stitch so that you can snip the thread very close to the buttonhole stitch.