Can I replace this bearing? (MTB head tube)
It's the most common kind of threadless headset. External cups, caged bearings. It uses a 5/32x20 retainer (bearing size x count), which again is the most common type and available online and at most shops for cheap. If you want you can also use loose 5/32" balls, which is more of a hassle but slightly better for headset longevity because you get more contact area and loose balls migrate around more freely and reduce pitting.