Make sure the XoT-S1 is powered up, and that it is not preforming a firmware upgrade.
Find the RESET button on the back the XoT-S1.
Press the button gently and you can feel and hear a click. Now hold the RESET button until the screen goes black, which takes 15 seconds. Release the button and wait for the screen to come back.
The screen should come back after about 30 seconds after it went black.
The XoT has now done a factory reset.
Clean the XMS from XoT-S1 details
Login to the XMS and look up the Device and XoT.
Go to the device page and delete the device and XoT.