How did one achieve upward, or downward, mobility in the group?

Men achieved this mobility by the size of their farm, yams in particular. Also exploits in war, wrestling and number of wives factor in. Lack of these things meant downward mobility.

Class is not dictated; the men of the tribe could move upward or downward based on their achievements and respect from the community;

"Reputation is extremely important to the men in the novel. Personal reputation is publicly denoted by the ankle bracelets men wear, which signify the number of “titles” they have earned. Reputation is based on merit – men gain reputation through bravery in battle, skill at wrestling, and hard work as seen through the size of their yam harvest. Reputation earns men positions of power and influence in the community as well as numerous wives. Okonkwo, the novel’s protagonist, is extremely concerned with reputation because he grew up with a father who was shameful and lazy. Okonkwo overcompensates by working tirelessly on his farm and taking every opportunity available to prove his bravery and strength."