On the login page there are two ways back in. Forgot password? takes your email and sends you a link to choose a new password. Email me a code instead needs no password at all: type your email, press it and enter the code we email you. If an email is slow, look in your junk folder.
The FullDiary app has the same Email me a code instead button. Its Forgot your password? opens the website's reset page, because the link finishes in your email.
To change a password you know, open Account & plan, then Your account, and use Your password; it asks for the current one first. If you signed up with Google, the same card says Set a password instead. A password lets you log in with your email as well, which is how the FullDiary app asks you to sign in.
The email you log in with is shown on Your account but cannot be changed there. Email hello@full-diary.co.uk and a person will help. A member of staff who forgets their password uses the same Forgot password? link.