When Plesk or another site is opened via HTTPS, a warning is displayed regarding an expired certificate.
How to renew this certificate?
Plesk offers multiple ways of renewing and using SSL certificates. It has Let's Encrypt extension that allows using free SSL certificates for domain and its aliases. The extension is capable of self-renewing the certificates, so there is no need to worry about manual renewals. See the following article to learn how to install and use Let's Encrypt certificates:
In case the certificate is purchased from some certificate center (for example from Comodo) and the certificate expired, get in contact with the certificate issuer and get the renewed version of the cert. After that, upload the certificate via Plesk > Tools & Settings > SSL/TLS certificates (server level) or via example.com > SSL/TLS certificates (domain level).
If self-signed certificate is used, generate a new one using instructions from the following documentation: Securing Plesk and the Mail Server With a Self-Signed Certificate