what is appropriate post-processing of the displacement product, like for example the masking of the incoherent values, for more accurate displacement measurements ? how we can implemented it on SNAP?
The easiest way is to terrain correct both the coherence and the displacement raster and stack them to have both of them in the same product.
Afterwards you can define a valid pixel expression for the displacement in the raster properties using the coherence band to set a threshold (e.g. coherence > 0.3). This will mask out all pixels with lower coherence than the defined threshold.
Some notes on this: Combine coherence and displacement bands?
If you want to use the masked raster outside SNAP, please also consider this point: