only pixel shaders can change textures in VRAM.
Pixel shaders only have read-access to textures. And their write position is pre-determined before pixel shader execution and not changable.(Hence pixel shader :) )
But, anyway, the fatest way is to just store a copy in local sysram. perform updates to that(And reads directly on the sys mem copy for v.fast performance) then just upload it. Uploading a 512x512 texture from sys ram to vid ram for example is v.fast.
And on modern cards you can actually set the gpu's draw buffer to that of the texture's, using pbuffers. This doesn't work on a *lot* of older cards, but it's the fatest method. (not supported in b3d)