how to filter or manipulate columns of csv file using another column using python

import csv
from datetime import datetime
import random

f=open("gh.csv","w")
csv.reader(f)
g=open("gh.csv","a")
k=csv.writer(g)
k.writerow(("age","ageband","gender","disease"))
for i in range(10):
    k.writerow((random.choice((range(0,100))),'40-50',random.choice(['male','female']),random.choice(['heartstroke','breastcancer','hypertension','hive'])))
for col in f:
    age=row[2]
    if 40<=age>=55:
      print '40-55'
    elif 56<=age>=75:
      print '56-75'
    elif 76<=age>=100:
      print '76-100'
    else :
      print 'age>20'    
f.close()


age ageband gender  disease
62  40-50   female  breastcancer
15  40-50   female  breastcancer
6   40-50   male    breastcancer
59  40-50   male    heartstroke
64  40-50   female  breastcancer
98  40-50   female  breastcancer
51  40-50   female  heartstroke
95  40-50   male    heartstroke
27  40-50   male    breastcancer
31  40-50   male    hypertension

after writing random data into csv file . is it possible to overwrite certain fields based on columns. for example if the age is 35 then the age band column should be updated as ''56-75'. and breast cancer should be updated only for females. please i need help as i am new to python. any kind of help is much appreciated