I removed the wall but my nav mesh seem not notice that change. What can I do?
And another question, why would one leg isn’t blocked by green space? Is something wrong with my static mesh?
Your answer is Here https://answers.unrealengine.com/questions/445801/nav-mesh-bound-limits.html
Well, I didn’t notice blocking volume for the wall that I removed. So the 1 question was resolved.
but Q.2 : Sometimes static meshes with different touch to the ground allow green nav mesh underneath, sometimes not .I used to have similar problem, but I forgot the solution. It was easy - one click, but I don’t remember what exactly. I would appreciate reminding. 
This Might Help Basic Navigation in UE4 — Vikram Codes
I know the basics. My specific issue is - some static meshes have “legs” or for example a wall with the open hole for a door. The open “door” must be green to let AI path through. But sometimes it is shown blocked, and it takes one click somewhere to allow that possibility.
Figured it out, In World Outliner Search for RecastNavMesh scroll down to Generations change the Cell Size, Cell Height, Agent Radius, Agent Height to 1 that should fix it, Work with those four things should help