Mainframe – file aid – edit VB file

HI,
I have a sequential file created as variable block. the record lenth max is 1000. (allocated as 1004)
One of the record in the file is 1200. i want to add some more char at the end and make it as 1250 char.
how i change the record length of a perticular record.
the record length of a record (first 4 char) is not visible in file aid.
pls give me your suggestion.