import os
import csv

with open('f1.csv') as f1, open('f2.csv') as f2, open('out', 'w') as out:
    reader = csv.reader(f1, delimiter='\t')
    writer = csv.writer(out, delimiter='\t')
    lines = set(line.strip() for line in f2)
    for row in reader:
        if row[1] in lines:
            row[1] = '123'
        writer.writerow(row)
os.rename('out', 'f1.csv')