Both live in the same place. Sign in, open My account, then Account details.
Changing your email
Enter the new address and save. Future order confirmations and voucher codes go to the new address. Orders you placed before the change keep their original delivery email, but the codes remain visible on the orders themselves.
Changing your password
You will need your current password to set a new one. Use something you do not reuse on other sites, since a reused password is the most common way accounts are taken over.
Locked out
Use the reset link on the sign-in page. The email arrives within a few minutes. If it does not, check spam, and confirm you are using the address the account was opened with.