Neck pain and handlebar adjustment

This bike has a threadless stem. Based on the picture, it seems the handlebar is already at the highest possible position with stock parts (spacers below stem, stem flipped up), and there is little adjustment in the form of spacers.

There are two possibilities for your discomfort. Either the handlebar is too far from the saddle, or the handlebar is too low. Most likely both.

In order to help your situation, buy a threadless stem with a high 35 degree angle. The spec doesn't specify the handlebar mount diameter, but I suspect a 1+1/8" steerer tube attachment, 31.8mm handlebar attachment stem is what you need.

Looking at the picture, the existing stem seems to be 6 degrees, perhaps around 90mm? The specs don't say this.

The 35 degree stems can be found as short as 60mm. Such a 60mm 35 degree stem moves the handlebar 46mm closer to you when compared to 90mm 6deg stem. It also moves the handlebar 12mm up. So much closer but not much more up.

By buying a 35 degree 80mm stem, you can move the handlebar 28mm up and 34mm closer to you, when compared to 90mm 6deg stem.

So, I suspect a 35 degree 80mm stem is what you need.

Of course, before knowing what fit is ideal, you might find a 80mm stem with adjustable angle a good starting point.

However, before adjusting the handlebar position, adjust your saddle height and move your saddle forwards/backwards as desired. A good starting point is knee over pedal spindle, but it's just a starting point. So, sit on the bike, with one leg forwards and one backwards on the pedal, have a thread with weight hanging from the front of your front knee. It should show a vertical line that is directly over the pedal spindle. This is the forwards/backwards adjustment. Before it, you need to get your saddle height correct. The easiest way is to gradually increase the saddle height. You know when it's too high.

The reason for adjusting first your saddle position is that any saddle adjustments will nullify any handlebar adjustments you can make. Seat adjustments are often free, i.e. the seat can be moved forwards/backwards/up/down with the stock seatpost. However, moving handlebar closer and higher will require buying a new stem.