Trasitioning from ArcGIS v100.6 to latest version

NACHIKETA

I am working on a project where we are using ArcGIS SDK .NET v100.6 , we want to transition to the latest version, we want to know whats new in the latest version and how many  changes we need to do .

these are the dlls we are using 
Esri.ArcGISRuntime
DotSpatial.Topology
Esri.ArcGISMap

SomeGISGuy

You really shouldn't have to do many changes. As a first step, I'd recommend that you upgrade to 100.15 and get things running there. There shouldn't really be anything other than just changing the version to do that, besides maybe some new minimum system requirements.
Next address any obsolete warnings in your build. Anything that was marked obsolete in this build, will have been removed from v200.0. If you have no obsolete warnings left, your move to v200.4 should be pretty smooth, again barring any minimum requirements like minimum supported Windows version or increased .NET version requirements.

You can see a list of release notes here:
https://developers.arcgis.com/net/release-notes/release-notes-for-200-4/

And click the "prior release notes" in the TOC to go through the history of changes:

This only applies to `Esri.ArcGISRuntime`. I don't know the other DLLs you're using.