Assuming that your unwrapped phase is “Unwrapped Phase_ifg_srd_VV_05Dec2008_24Apr2009”
and your reference height is 15:
Right-click on your unwrapped phase open Band maths and enter
Unwrapped Phase_ifg_srd_VV_05Dec2008_24Apr2009 - 15
If the measured value is negative, add it instead.