How does one change circuit breaker? It doesn't click into place, it's just very mushy & loose.

You may not have to change the circuit breaker. I am wondering if it tripped or did you turn it off on your own? Or, if it did trip is whatever made it trip in the first place is still causing it to trip?

It's not hard to install a new one.

For installing a circuit breaker please see the following links:

Installing A Circuit Breaker

Installing Circuit Breakers

CIRCUIT BREAKERS