In an emergency, use the stairs, not the elevator.
Literal
Emergency [possessive-の] case [topic-は], elevator [copula-ではなく], stairs [object-を] use [imperative-なさい].
ではなく means 'not X but (rather Y)' — the negative copula で+は+なく used contrastively. This is a standard safety instruction found on signs in Japanese buildings.