Quick question: can you raise up the handlebar on the Ozark Trail G1 Explorer drop-bar?

My son has been using the Ozark Trail G.1 (large size I'm 90% sure) daily since he got it for Christmas but he's telling me he needs to raise the handlebar up higher for comfort as he's grown.

I don't know bike mechanics and this one came pre-assembled from Walmart so I didn't get any experience putting it together.

I tried to remove the screw at the top of the handle bar stem to see if I could lift it up but had no luck and didn't want to break anything for forcing it too hard.

I figure you folks would know since the bike is so popular here: can I lift up the handlebar stem higher for a growing kid or is it in a fixed single position?

If I do need to replace the stem with a longer one, do you have any suggestions for a good one?

Thanks for your time!