If you are in the USA, then it is my opinion that Garmin support is pretty helpful. But you have to call them on the phone, do not email them. There can be a long wait time, but when hold times are long, their phone system will allow you to key in a call back number and they will call you back just as if you waited in the queue.
They have an "Out of Warranty" flat rate repair. It currently shows $99 for the Edge 520. You'll get a re-furb'd unit though, so make sure you save whatever you want off the other if they require you to send it in.

As for those sites that show replacing batteries in various Garmin Edges, it's something you really have to consider how well you think you might be successful. I've seen many posts on Garmin's forums and other forums from people that tried and screwed up.