Do you actually need to reset the password, i.e. did you forget your password?
If you still have your password and can successfully login to your plex.tv account, then you only need to change the password here: https://app.plex.tv/desktop/#!/settings/account
Don’t forget to tick “Sign out connected devices after password change” while doing so.
Afterwards, you need to re-claim your server, in order to regain access to it.
