Custom zener diode in LTSPICE
A quick way of adjusting the zener voltage of a spice model, is by creating an alias to it and modifying the properties you need, e.g.:

The zener diode D1 uses a model called MYD, which is nothing but a copy of the built-in zener diode BZX84B8V2L but with a different zener voltage (5V in this case). 



You can also create a model from scratch using the directive "D":