Yarn over and bring the yarn through the stitch hole.
④
Now we have 4 loops on the hook. We yarn over and bring yarn through 2 loops each time. Until there is 1 loop left on the hook
⑤
Then we can crochet with the new yarn. We insert the hook in the next stitch and do a single crochet. We can hide the tail by putting the tail on the hook. The old yarn can be hidden together.