Please help - how to change neckline

I am knitting the twinkle sweater from the new issue of vogue knitting. It calls for a cowl neck but I would like to change it from a cowl neck to a turtleneck. How would I do this?

At the end of the pattern, where it says “For All Sizes… Work even on 40 (44, 46, 46) sts…” You’ll need to decrease down to about 24 - 30 sts, depending on how much ease you need. Then knit for about 3 or 4 inches before binding off.