Can I replace upgrade main breaker without changing wiring? (within panel limit)
It it unlikely that you would be able to put in a 63A breaker. The available size would be more likely be a 60A breaker. You also need to consider what feeds this panel and what size wire brings power to this panel. 

1) If the panel is feed from the utility company via a power meter then the MAIN breaker would want to be sized according to the wire gauge from the meter and within the ratings of your panel. This would require consultation with the utility to determine what the delivery capability is in Amps.

2) If this is actually a sub-panel that is fed from a main breaker panel elsewhere on the property then there should be a source circuit breaker in that panel that protects the feed wires to the sub-panel. There would be no point to install a larger ampacity breaker in the sub-panel than what is in the main panel. If that main panel breaker is 30A for example and the connecting wiring is 10AWG then it would not be safe to update any part of the sub-panel power delivery to higher capacity without upgrading the wiring to a proper increased size and possibly changing the feed breaker in the main panel to a size that is compatible with the wiring installed.