dictionary = {}

with open("password.txt") as file:
    for line in file:
        e, p = line.strip().split(", ")
        dictionary[e] = p

email = input("Enter email: ")
password = input("Enter new password: ")

dictionary[email] = password

with open("password.txt","w") as file:
    for e,p in dictionary.items():
        print(e,p,sep=", ",file=file)

you@example.com, old_password
noone@example.com, uzt(647zt2(/r
lastone@email.com, zzzzzappp!
youtoo@example.com, password
xkcd@example.com, ok horse battery staple
weall@example.com, topsecret