Final answer:
In 'A Real Durwan,' a new light fixture is installed in the stairwell, representing the modern changes affecting the protagonist, Boori Ma.
Step-by-step explanation:
In the short story 'A Real Durwan' by Jhumpa Lahiri, the item installed in the stairwell is a new light fixture. This installation is part of the upgrades made by the residents of the apartment building which inadvertently leads to the displacement of the protagonist, Boori Ma, who is the durwan (doorkeeper). The introduction of the light fixture symbolizes the changes impacting Boori Ma's life, reflecting themes of modernity and displacement.