Place the server in maintenance mode in the monitoring systems (for example, SCOM).
Install Cumulative Updates best practices You now must use either /IAcceptExchangeServerLicenseTerms_DiagnosticDataON or /IAcceptExchangeServerLicenseTerms_DiagnosticDataOFF for unattended and scripted installs. Note: The previous /IAcceptExchangeServerLicenseTerms switch will not work starting with the September 2021 CUs. Be sure to save this information so you can easily re-apply the settings after the install.
Any customized Exchange or Internet Information Server (IIS) settings you made in Exchange XML application configuration files on the Exchange server (for example, web.config files or the file) will overwrite when you install an Exchange CU.
Uninstalling the new version completely removes Exchange from the server.
After you upgrade Exchange to a newer CU, you can’t uninstall the new version to revert to the previous version.
Good to know before installing Cumulative Update
Check Exchange Servers report for vulnerabilities.
Check that Exchange Server is up to date.
Install Cumulative Update on all Exchange Servers.