Unfortunately there is not a way to only increase the volume of the two upward firing speakers. But this is something many Arc users have requested. Do you have high ceilings?

You might want to tilt the arc slightly to get the reflection off the ceiling hitting your seating area better.

I wish Sonos could release an update to make the upward firing speakers louder. My ceilings are not high, I live in a apartment and my ceilings is 9’ high.  I bought a high tv stands and mount brackets to raise the sound bar higher. The Arc is sitting 36” from the floor. 
 

I notice you have a ceiling fan. I’m sure that doesn’t help either.

I had no issues with my previous soundbar and the fan being in the way. With my previous soundbar I was able to control the loudness of the upfiring drivers, side drivers, and center channel.

Have you seen this video?  You could be too far from the bar, you would need to move your listening position closer, and tilt the bar ever so slightly forward.

How to place the Sonos arc for the best Atmos effect.

If it helps, I have 8 ft ceilings and my Arc is 2 ft from the floor. My Atmos “sweet spot” is sitting about 7 ft away from the Arc. The height effect isn’t dramatic, but it’s about what I expected from a couple of speakers that rely on reflecting sound off of a ceiling. But, yes, it would be nice if Sonos gave us the ability to adjust the volume of the upward firing speakers up a couple of notches.