If you still have access to the old phone or to your authenticator app: go to Account, remove the registered device and add a new one.
If you have lost access — a replaced or wiped phone, an authenticator app that was not restored — there are two routes.
If you have Google or Microsoft connected to your account, sign in that way. It does not require your Sigill one-time code. Once you are in, go to Account, remove the old two-factor device and register a new one. This is by far the quickest route, and a good reason to connect one of them before you need it.
Otherwise, contact support@sigill.tech. You cannot restore it yourself, because signing in requires the code you no longer have, and administrators at your organisation cannot reset it for you. We will verify who you are and reset your two-factor authentication, after which you set it up again at your next sign-in.
Note that resetting your password does not help on its own — a one-time code is still required afterwards.
To avoid the situation altogether: register your authenticator in an app that syncs across your devices, or store the setup key somewhere safe when you first configure it.