Ask any electrician about replacing circuit breakers and the first piece of advice you will get is to turn off the mains before you start working. This is a vital step to avoid electrocution injury, but there is still an electrical hazard in the panel itself because the wires going to the mains still carries current. Your hot bus bar may also have current, and you will need to be especially careful if you have never opened the circuit breaker panel before.
Before attempting any circuit breaker replacement, ask yourself how you will accomplish the job with all the lights out. Some people believe a flashlight will do the trick, but it is very important to have a steady light source such as a battery powered lamp or "electric lantern" that you don't need to hold. You need to be able to clearly see the potential danger spots in your electric panel at all times. Accidentally touching the live wires from the power company can cause a fatal shock. Being able to see and avoid them at all times is crucial. Do not attempt to install new breakers unless you can clearly see all parts of the panel and use both hands to do the work.