I live in a condo , one of the circuit breakers had to be replaced because of flickering lights, but it has never tripped and had to be reset. It was a older harder to find one, so the only one the store had that would fit was 20 amps and my old one was 15 amps, is this safe? The man at the store really couldn't answer my question as he was not a electrician.

NO, it is certainly not safe. You must replace a circuit breaker with the same size.

Do you own this condo or rent?