Both are in Settings → Danger Zone → Security Actions.
Change your password: tap Change Password, enter your current password and the new one twice. A new password needs at least 8 characters, with an uppercase letter, a number and a symbol. When it's changed, your other devices are signed out and we email you to confirm.
Change your email address: tap Change Email, enter the new address and your current password. We send a confirmation link to the new address; it works for 30 minutes. Once you confirm, we let your old address know.
For your security, both need a sign-in from the last 30 minutes. If it has been longer, Nixyum first asks you to confirm it's you.
Signed up with Google or Apple? You won't have a password yet: use Set Password in the same place, and we'll email you a link to create one.