I'll try to walk you throug
Let say I Have a main form called Form2, and a subform Called Form1.
In the sub form I have a field called  mass and after I update that field I
Run a refresh on another field Called MyField
I'll create a function in a module
function aaa()
  Forms![Form2]![Form1].Form![MyField].requery
end function

in the after update of the field mass I'll call that function

Private Sub mass_afterapdate()
   aaa
end sub

I'll also call this function on the after update event of the field on the
main form.