Answer
Inverse action: descend down the staircase
Work Step by Step
In loose terms,
let f(being downstairs)=get upstairs
(climb the stairs)
We want the inverse to be defined on being upstairs
g(being upstairs )=get downstairs
So,
g(f (being downstairs)=being downstairs
and
f(g( being upstairs )= being upstairs
[ f(g(x))=x and g(f(x))=x ]
Inverse action: descend down the staircase