Use the following link to login to your Office 365 mailbox.
Click on your profile button in the top right corner of the window then select Open another mailbox...
Type the email address of the mailbox you wish to change the settings of. Click on the directory listing below the text box then click Open.
A new tab will display the Inbox of the shared mailbox.
Click on the settings icon in the top right corner of the window.
From here you can modify the settings of the shared mailbox such as Automatic replies and Display settings.
For more options use the search box. For example, to create a rule type 'rules' into the search field.