Insert the LH needle from front to back (away from you) into the stitch below the one on the RH needle.
Make sure you have the correct side (either knit or purl) facing you.
Slide the RH needle out of the stitch and gently pull on the yarn to free it.
Your work won’t unravel, because your LH needle has secured the stitch below.
Repeat these steps, stitch by stitch.
Continue until you reach the point of your mistake.