how do I modify Texture2DArray values in shader

In Direct3D 11.1 you can use unordered access views (UAVs) for read-writable textures in all shader stages. So you can add an RWTexture2D to your vertex shader and use it to store values as the vertices are shaded. However, this functionality isn't available in Direct3D 11.0 or earlier, and 11.1 is only available on Windows 8.0 or higher.

If you're limited to Direct3D 11.0, you could implement your displacement operation using a pixel shader and doing a standard full-screen pass on the texture containing the positions. Another possibility (though likely slower) is using a compute shader. In either case, you would do this as a pre-pass before actually drawing the displaced mesh.

You cannot moddify textures in the pixelshader that way, the only way to do that is to render to a texture.

Direct compute offers you the possibility to read and write data to a texture on demand. you could achive your diserd effect by having a Direct compute pass.

Here is a good link to msdn about compute shaders

Since it's vertex data you want to store somewhere you also have the option of using stream out from either the vertex or geometry shader.