Adding length

I have a cardigan sweater pattern that ends at the waist. How do I make it longer to make it end about thigh high?

A hand knit sweater? You could take out the bind off/cast on and put the resulting stitches on needles. Then knit down with either the same yarn or something of similar weight till it’s as long as you want. If it’s fitted you may need to do some increases for the hips.