How to add length to finished garment

I have finished a baby cardie only to realise that I mixed up knitting between 2 sizes so the body of the cardie is too short. It was knitted from bottom up, can I unpick the rib and add length here?

Welcome to KH!
Any possibility of re-knitting the top above the armholes?
If not, you can pick up stitches along the bottom rib and extend it a bit or you can use this technique to cut the sweater at the join of the rib and body. Then you would knit down as much length as needed. I’ve done this several times and it works beautifully.

What pattern are you following?