How to change the Quality of texture in run time ?

You can create an Atlas which contains multiple textures. Then you can create $$anonymous$$aterial of that atlas and access the different textures by changing the tiling and offsets of the material

Do u have demo package of Atlas ? please share

Unity put them automatically into an atlas if you have the pro version and give the same Packing Tag (in the sprite import settings tab) to all sprites supposed to be in the same atlas (otherwise more atlases are created). This only works for sprites not with normal textures.