MCB Breaker

Just to preempt all those who are thinking of flaming you...

MCB in North America is a designation meaning Main Circuit Breaker, usually associated with a Panelboard or Motor Control Center. 500A would be an MCCB (Molded Case Circuit Breaker) as far as the design goes.

MCB in the rest of the world means Miniature Circuit Breaker, a small device that is 100A and below. I said this because people have a tendency to jump all over perceived errors in here, I think this is just regional semantics.

Your use of the term MCB to mean a large breaker makes me think you are in North America somewhere. That being the case, adjustable trips on standard breakers are not allowed here, only electronic circuit breakers are permitted to have this, and they are uncommon in that size range on panelboards and MCCs.

But some MCCBs in the 300-500A range have what are called "interchangeable trips" meaning there is an entire replaceable block on the bottom half of the breaker that can be removed and exchanged for one of a different trip rating at or below the frame size of the breaker. So if you have a 600A frame breaker with a 500A trip module, you may be able to exchange that with a 300A trip module. But without knowing exactly what you have, we have no way of advising you further. If you breaker is not of the interchangeable trip design, you must replace the whole thing. So like TonyS said, "Get your cheque (check) book out".