The basic synthax is:

Forms![MAinFromName]![SubFormName].Form.[ControlName]

so in your case you could do something like:

Forms![Site_info_frm]![FAsub_info_frm].Form.[File_Path]="NewValue"

Forms!Site_info_frm!FAsub_info_frm.Form!File_path

Note, though, that depending on how you added FAsub_info_frm as a subform on
Site_info_frm, it's possible that the subform control on Site_info_frm might
be named something other than FAsub_info_frm. If the subform control on
Site_info_frm is named something other that FAsub_info_frm, us the name of
the subform control instead.

You might find it useful to read
http://www.mvps.org/access/forms/frm0031.htm at "The Access Web"

On my website (www.rogersaccesslibrary.com), is a small Access database
sample called "SubformReference.mdb" which illustrates how to handle a lot
of reference issues on subforms and sub-subforms.