Question about Material.mainTextureOffset

Hi, there,
I am wondering whether we can use Material.mainTextureOffset to change the texture outside the function Update()?
I mean I want to import image as texture at runtime and adjust the offset of the texture image. But when using the Material.mainTextureOffset function in the OnGUI(), I found the offset of the texture is always reset. Is there a way to do what I want with Material.mainTextureOffset?

Here is the code:
image_position = render.material.GetTextureOffset('_MainTex')
if GUI.Button(Rect(GetPositionX(3) - 10,GetPositionY(3),42,42),"Up"):
image_position.y -= 0.1
render.Material.mainTextureOffset = Vector2(image_position.x, image_position.y)
#I also use render.sharedMaterial.mainTextureOffset = Vector2(image_position.x, image_position.y) here,
#but it does not work well either.

And I also try the function Material.TextureOffset, it does not work either.

Thanks in advance!