Dbu the file you want to change.
Press F15 (Shift F3).
Under the Replace section, put the name of the field you want to change and its new value. Further down you will see an option for how many occurrences to process. Put a 2 there to process all occurrences in the file. The last line will ask you how many records you want to search. I generally fill this line with 9's.
See example below:
Type choices, press Enter. A/O: A=AND O=OR
Format... BLKSHIPTO Operation: GT LT EQ GE LE NE NG NL CT NC
Value: Field, Number, 'Characters' or X'HEX'
Search... Field Name
A/O *RCD *RRN From To Op Value, *DECERR or *NULL
Replace.. Field Name From To Value or *DELETE
Create subset of records . N Y=Yes, N=No
Include deleted records . N Y=Yes, N=No
Output results to file . . N Y=Yes, N=No
Occurrences to process . . 2 1=Next, 2=All, 3=Previous
Kind of match . . . . . . 1 1=Same case, 2=Ignore case
Print results . . . . . . N Y=Yes, N=No
Records to Search. . 999999999 & Progress Messages every 1000 records
F3=Exit F4=Fields F9=Toggle SQL/SEARCH F16=Search F21=Submit to Batch