From my manual:
Replace by Location
The syntax of the REPL parameter when used to unconditionally replace data in a given location is:
REPL=(location,[dupl]new-data)
location : Starting location of the data to replace. Any valid actual or relative location can be used.
dupl : Optional duplication factor that defines the number of times File-AID repeats the new-data, starting at the specified location.
new-data : New data to place in the specified location. Any data format may be entered.
Example 1:
$$DD01 COPY REPL=(4,Cā6ā)
Example 1 copies the input dataset and replaces the data in location 4 of each record
with a character 6.