The ZEN Thermostat contains a precision temperature sensor and should not need to be adjusted. It's possible that your old thermostat was less accurate.
However if you would like to offset the temperature as seen on the display, remove the device from the wall and press the CONFIG button five times to enter the advanced menu screen. Once you are in the advanced menu, navigate using the right arrow until you see T0 appear. You can now adjust the offset up or down by the number of degrees you require. Once you have selected this, press the centre of the device to confirm your selection and your ZEN Thermostat should blink to confirm your selection. Return the display to the wall and your adjustment will be saved.