When Plesk or a Plesk website is opened via HTTPS, a warning regarding an expired certificate is displayed.
How to renew this certificate?
Plesk offers multiple ways of renewing and using SSL certificates.
Plesk has the SSL It! extension that allows using free SSL certificates for domains and their aliases. The extension is capable of self-renewing the certificates, so there is no need to worry about manual renewals.
More details on how to install and use free Let's Encrypt certificate for a domain can be found in this article. For Plesk itself here.
Note: There is an exception here. If a wildcard certificate is used for a domain and DNS is not managed by Plesk, it will be required to update the certificate manually as per the article due to the limitations on the Let's Encrypt side.
In case a certificate is purchased from some certificate authority, for example, from Comodo and this certificate has expired, get in contact with the certificate issuer to obtain the renewed version of the certificate. After that, upload this new certificate for a domain as described here. For Plesk itself follow the steps from this article.
If a self-signed certificate is used, generate a new one and upload it as described in this guide.