you need SLC products for this, one polarization (preferably VV) is sufficient.
The steps to generate an interferogram are described in this tutorial: http://step.esa.int/docs/tutorials/S1TBX%20TOPSAR%20Interferometry%20with%20Sentinel-1%20Tutorial.pdf
To convert it to absolute phase changes, the interferogram must be unwrapped.
Unwrapping itself is demonstrated here: SNAPHU Unwrapping
Then you import the unwrapped phase back into SNAP (described here: Problem in Snaphu Import section)
Lastly, you perform “phase to displacement” to get the change along the line of sight.