Welcome to KH!
In row 1 there are paired increases (yarn forward or yarn overs) and decreases (k2tog or skp). There are 2 increases in a stitch (you could use a knit front and back) one at the beginning and one at the end of the row. These two increases account for the stitch number change from 37 to 39.
You might put in stitch markers after the first 10sts, after 12sts and then after the next 12. There’ll be 3 sts left at the end. Do this before you work the row and it should help keep you on track. It might also help to write out the row for your size since there are so many directions that can be skipped (a 0 for the 4th size).
There’s really only one sweater project on Ravelry but no errata noted.