- (BOOL) writeField: (NSInteger) recno fieldno: (NSInteger) fieldno;

Function writeField() writes a single field from internal memory block to the file.
You have to fill the memory with necessary values using setValue()

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