Function ModDataFile(Name:String;Number:Word;FName:String):Word;

    This is similar  to  ModData.   It  will  overwrite  a  data item (it
    need not have been  inserted  with  NewDataFile,  it  may  have  been
    inserted with NewData) that is the same size at the file specified.

    No memory check is made so the  user  must  ensure there is 2 kb heap
    available plus heap  space  to  load  the  entire  file  into memory.
    Thus 66 kb heap is the most the procedure will require.