Outlook has changed it's interface, so if you're looking for the old control panel.... go to File->Account Settings ->Manage Profiles. Select Email Accounts (top of the list). Once there, the 'old' control panel is shown (the one that we are used to). It's such a nuisance when they change things!
Here are the detailed instructions for accessing the 'Advanced Settings' in 2018.
Within MS-Outlook, go to File->Accounts Settings ->Manage Profiles
If you get a message 'Do you want to allow this APP to make changes to your device?" click YES.
Next you will get a popup window with some options, choose "Email Accounts".
The next popup window will give you a list of the accounts that you have setup within Outlook. Select the account that you want to make changes to, you can double-click it.
The final screen will be the screen that you are used to seeing, and will contain the "More Settings" button where you can adjust settings such as 'Leave a copy of messages on the server for x many days".