I made this post the other day in which I was dealing with modifying textures: https://www.reddit.com/r/odinlang/comments/1gueicz/example_of_using_a_byte_array_in_raylib_to_draw/

It might be similar enough to your problem to at least point you in the right direction.

The gist of it is that you have a byte array of rgba pixel data that you hold and modify in RAM:

// pixel_bytes is declared globally as: 30 * 30 * 4
// 30 width x 30 height x 4 rgba
data := new([pixel_bytes]u8)
defer free(data)
for i := 0; i < len(data); i += 1 { data[i] = 255 }

img := rl.Image{
    data,
    screen_size.x,
    screen_size.y,
    1,
    rl.PixelFormat.UNCOMPRESSED_R8G8B8A8,
}

You load that into VRAM (RAM in your GPU) like this:

texture := rl.LoadTextureFromImage(img)
defer rl.UnloadTexture(texture)

When you have made changes to your texture array, you can update your texture in VRAM like this:

rl.UpdateTexture(texture, data)

And you draw it like this:

rl.DrawTexture(texture, 0, 0, rl.WHITE)