When changing the rear brake caliper on my 2012 LR2 I didn't unplug the electric hand brake.
Consequently the piston has fully extended

How do you get the piston back into place

Put it EPB into Maintenance mode.

Then try and get the piston back in.

You may have to rebleed the brakes if its come right out.

Putting electric hand brake into service mode before changing pads.
Procedure is: 

Ensure its in park or neutral and the park brake deactivated and ignition turned on. 

1. Press and hold the parking brake switch in the release position. 

2. Wait 2 seconds 

3. Press and hold the accelerator pedal in the wide open throttle position. 

4. Wait 2 seconds 

5. Turn ignition off and on immediately whilst still finger on park button and foot on throttle. 

6. An audible noise will confirm service mode, it will also be displayed on dash. 

You will hear the brake motors activate for a while. 

Deactivation 

with ignition on. 

1. Press and hold the parking brake switch in the apply position. 

2. Wait 2 seconds 

3. Press and hold the accelerator pedal in the wide open throttle position. 

4. Wait 2 seconds 

5. Turn ignition off and on again. keeping finger on park button and foot on throttle. Jules