After about a year of usage one of my Estimote iBeacons ran out of battery power this spring. I couldn't find any proper instructions explaining how to replace the CR2450* battery inside, but I did find some guidance in a comment on an Estimote blog post.
...the best and easiest way to cut the Beacon is a vertical cut across the top of the enclosure.
I followed the advice, and I have to say it worked out well enough.
Using a razor blade I made a cut down the middle allowing me to, with some effort, wrestle the actual beacon hardware out of the enclosure and replace the battery.
The beacon enclosure does carry a visible scar from the operation, although I doubt you'd notice unless you knew to look for it. I also suspect the device is no longer weather resistant. But it is, once again, a fully functioning iBeacon!
Extending battery life
After briefly playing around with the power settings, the new battery in my Estimote beacon has a life expectancy of 32+ months.
*Estimote beacons shipped in May 2014 and later use a bigger CR2477 battery.