I have received a new Mattermost Enterprise License and want to upgrade the existing system with said license.
You can install the license as soon as you receive it. There is no begin-date to a license, only an expiration date. Updating the license ahead of the expiry date has no negative side effects.
Updating the license can either be done from the System Console by removing the old license and uploading a new one. Or through the cli by running the mattermost license upload command, or through mmctl by using the mattermost license upload command.
When replacing the license through the System Console, the configuration persists even if you remove the license file. So usually there will only be a limited time of a few seconds where the system will not be an Enterprise deployment. Using the cli/mmctl will circumvent this issue.