How can a demon slayer become a hashira? What are the requirements? Is there any exam conducted to become hashira? No one has defeated 12th upper demon before, so I believe that is not the condition.
You are not needed to kill a upper demon to become a Hashira if you can fulfill the other condition.
To be a member of the Hashira, a Demon Slayer must be among the strongest combatants in the entirety of the Demon Slayer Corps. There are two known methods (both extremely difficult) that allow one to prove that they are worthy of becoming a Hashira: the first method is to accumulate a demon kill count of at least 50 as a Kinoe-ranked Demon Slayer, or slay a member of the Twelve Kizuki. The second method is being a Tsuguko of a Hashira, which requires immense talent.