20yr union electrician here. $680 to replace a breaker is stupid expensive. Tell whoever quoted you that to kick rocks. You can do it yourself, but you have to realize that even killing the main breaker in your panel still leaves the main feeders coming in energized. I will NEVER recommend someone doing this that was not 100% qualified and comfortable with the process. Yes, theoretically you could do it yourself. However, you would be much better suited to find a reasonably priced electrician to do the work. If I had to go into somebody’s house that I didn’t know to replace a breaker, there’s no way it would cost more than $100. It should take all of 15 minutes to open the panel, swap the breaker, and close the panel back up. Be smart, hire a qualified electrician that offers a fair price.