Call your power company and schedule the gold ring and meter's removal. Once the power company representative unlocks the ring, have him pull the meter out of the socket. You could also request that the power company put a blank cover over the meter socket to prevent anyone from getting shocked while it's open.
Disconnect the wires feeding the main breaker from the meter socket. Disconnect the wires feeding the panel buss, sometimes there are no actual wires, but instead are metal stubs that are attached to the meter's buss and connect it to the main breaker.
Remove all bolts or screws that may be holding the main breaker into the panel box. Remove the main breaker. Install the new main breaker into the panel. Reattach the bolts or screws, if there are any. Reconnect the wires and stubs to the new main breaker, assuring that the connections are tight.
Call your power company and ask for the representative to return and reinstall the electrical meter and lock ring. When the representative returns, show him the work you did so he can verify that everything is correct.
Turn your main breaker back on. Power should be restored.