You can only compute vertical displacement combining both ascending and descending orbits or potentially by using a separate Ground Control Point vertical height change time series. You can search on these forums for more posts on how to exactly compute this or look in to the referenced papers in my thesis.
This post may help you: StaMPS-Visualizer, SNAP-StaMPS Workflow
It explains how to compute vertical displacements from only 1 orbit, please be aware that if you are not using 2 orbits the result will probably be incorrect.
Best Regards,
Gijs