Login to https://webmail.eppihost.com/ with your username (usually your full email address, e.g. user@example.com) and temporary password.

The first time you login, you will be asked to change your password. Do so, and save your new password in an encrypted password vault (something like KeePassXC).

After changing your password, select Settings, then Identities, then your username. Put in the name you'll use in outgoing emails for "Display Name". If "Email" is not already set to your email address, set it.

Finally, send a test message from your new email address to an external account, and a test message from an external account to your email address, to confirm everything is working as expected.