Need smaller sz than pattern hmmm

I have never been brave enough to try to adjust a pattern to suit me, I have no idea how to do that. I recently knit an adorable pumpkin hat for my neice. (Fibertrends Patrick’s Pumpkin) I have a new nephew on the way and he will be a newborn for fall. I would love to make the hat for him but the smallest sz in the pattern is 19 inches. Is there a way to adjust my pattern to make it smaller or should I just go with one of the many other newborn pumpkin hats I found in my websearch all of which are ok, but not exactly what I want.

You could use the smallest size and thinner yarn and needles, if possible.