Put your left needle into the stitch below the one on the R needle, push the st on the needle off of it and pull the yarn. Repeat. Or just push the right needle back so those sts are free and gently pull the yarn out of them and put the left needle into them.
I insert the needle from the front on a purl stitch done in error, bring it back on the left working needle and pull the thread thru. On a knit, insert the needle from the back and bring it back on the left needle and pull the yarn thru the loop.