I don't know if it's meant to be user replaceable, but the fan housing (shroud? whatever it's called) looks removable.
The fan by itself is a little under 30mm in diameter (~27mm -blade to-blade, non-precise measurement with a ruler, with the board under plastic). I'm tempted to remove it, but I don't have all my parts in yet, so I can't check temps or noise. I would love to replace it or 3D print a replacement housing with a baby Noctua fan (40mm).
EDIT: Confirmed. Easy remove two screws, the fan housing comes off and it's just the heatsink - or bottom half of the heatsink, I should say, as the fan housing is also metal and likely transferring some of that heat - and m.2 slot underneath left on the motherboard. Able to look at fan unit and not just the blades now, it's a 30mm, at least by frame size (30mm x 30mm). Everflow R123510BM, 0.18AMP.
Oddly, it's listed under the "35 x 35 x 10mm" section (nevermind, I'm dumb: it's listed under both sizes) of Everflow's
datasheet. It's just a few more bracketed screws and tape attaching the Everflow and its cable, respectively, to the assembly so it's an easy removal process to get rid of the stock fan to replace it with another 30mm or cool the chipset passively.
40mm Noctua (NF-A4x10 FLX) should be a massive improvement - at least for noise, though on paper the airflow also looks like good upgrade; I'm going to 3D print an adapter after I get the fan, and use it if it turns out okay.
Stock fan (Evercool 30mm): 31 dB(A), 2.50 CFM, 2.99 mm H2O static pressure, 7500 RPM
Noctua 40mm: 17.9 dB(A) [12.9 dB(A) with the Low-Noise Adapter], 4.83 cfm (3.89 cfm with LNA), 1.78 mm H2O static pressure, 4500 RPM (3700 RPM with LNA)
Unsurprising that 30mm fans are noisy little bastards - it's their nature, they can't help it
- but just going to 40mm is a surprising jump. Virtually silent, judging by specs and Newegg reviews.