Presently passwords and username cannot be edited.
just to clarify for other people, this is editing passwords and usernames that are saved in Orion, not editing website passwords through Orion.
I've seen it on mobile but it seems desktop version is missing the UI to add/update/remove passwords/keychain entries.
(If it's there it's so hidden I can not find it and needs a more prominent placement)
Duplicate of https://orionfeedback.org/d/1012-username-password-edit