- (BOOL) writeRecord:(NSInteger) recno;

Function writeRecord() writes a record from internal memory block to the file.
You have to fill the memory with necessary values using setField()

{
    // your code
    for( NSInteger row = 0; row < [dbf recCount]-1; row++ )
    {    
        [dbf readRecord:row];
        // change data
        [dbf writeRecord:row];
    }
}