The following is quick-guide which outlines the two part step in performing a Factory Reset (Hard reset) to a Leica Zeno FLX100 (Regular/Plus/Tilt). 

• Hard Reset

• Firmware Upload

Factory Reset on a Leica FLX100 Plus.pdf

The firmware is always available in MyWorld under Downloads, GIS Asset Collection, FLX100, Software.  If you just need to perform an Firmware update (Upload FW on a FLX) the following is a quick guide:

How to upload firmware to the FLX100 plus.pdf