How do I change the login details email a customer gets when I create their account?

That email is the New Login Credentials template.

1. Go to Settings → Web Settings → Email Templates.
2. Type Login in the search box and click Edit on New Login Credentials.
3. Change the Subject (required) and the Email body. Use the Insert: chips to add the customer's name, email, password and your store link.
4. Click Save changes.

It is sent when you add a customer with confirmation to send a mail ticked, when you click Update & send login details on a customer's password, and when a guest checkout creates an account.

To stop it, click its Active badge on the list to make it Inactive. Editing the content depends on your plan.