I don't have official instructions, but I can give you some info....if it's still relevant over a year later....
The battery doesn't have the usual brand and serial number info on it (at least not on the exposed side when you crack it open - I didn't pry it out because I was just trying to fix the connection on mine). What it does have is the following:
+ JK 454872
3.7 V 1700mAh
That middle line is the specs of the battery - 3.7 volts and 1700 milliamp-hours. No wonder it lasts at least 12 hours per charge.
The screws at the bottom are what keep you from getting in - they're using Apple's proprietary 5 pointed star screwdriver - the same thing Apple uses to lock down the iPhone and such. They're threadlocked with a low-strength adhesive, so they'll take a bit more force to crack open than you'd otherwise expect.
You can then pry open the case from the bottom - the back pries up from the front. Note that the battery is mounted on the back and the connector is at the bottom right corner, so you'll want to do this very carefully. As with the screws, there's also a low strength adhesive (probably meant more to keep dust and fluids out, but it's easy enough to overcome.
Once there, you'll need to disconnect the battery from the circuit board on the front half. There's a very small Phillips screw holding a little bracket in place over the mount, which you'll need to disconnect. I was unable to figure out how to disconnect that mounting system - fortunately, I was borrowing the Apple screwdriver from a guy who does phone repair, and he got it disconnected and reconnected for me. (Turns out that wasn't the problem with my model, sadly.)
Once the battery's disconnected, you'll have to pry off the old battery from the back panel of the case (looked like more adhesive there), then put the new one in and reverse your steps. You won't be able to redo the adhesive, so it'd probably be a little more vulnerable to humidity, being dropped in water, etc, but most people are pretty careful about giving their electronics a bath anyway, so probably not a big concern.
I found a video showing how to disassemble the X1 on Youtube that might be helpful if you want a visual, or want to know what to expect when you crack yours open. No audio, but it's something.