How do I change a bad multibreaker?

Are you talking about a double breaker that is tied together? If so, it's simple. Kill the power supply to the panel either through the main breaker or the disconnect. Switch off the breaker, remove the breaker from the panel with the wires attached, remove the wires from the old breaker, install wires in new breaker, make sure the new breaker is switched off, install the new breaker onto the panel, then turn it on. Now supply power to the panel. Done. You can do this without killing the panel... but I would not recommend such a procedure. So just kill the panel and go back and set all your clocks, etc.

If this is the main breaker you're replacing, and there is no disconnect, hire a pro.

If it is a double 240 volt breaker. It will have 4 screws. 2 center is 1 circuit, the outer 2 is the other circuit. Duplicate what was there. Good advice above.