Hi,
it is actually impossible: in the tutorial, the OBJ Temp is not associated with an [b]input widget[/b] (1) and is not raised to a [b]top-level interface variable[/b] (2), so it must be perfectly possible to edit it through the Block Details pane (or even the Block Summary pane), both prior to launching and during the simulation (it is a manipulated variable !).
In case you modified the original tutorial by either using an input widget or a top-level interface, then the OBJ Temp field in the Block Summary pane will be [b]disabled[/b] (which is intentional of course) and it can only be modified through the input widget, in the former case; or through an input file (if the top-level interface is associated to a file) or through the Block Summary pane for the plant level (not the ASU block level!), in the latter case.