Log into Office 365 via Webmail.

Once logged in open Outlook on the top panel.

In the top right of the screen click on your profile picture and then select Open another mailbox.

Type in Mailbox name that you are looking to edit and press open.

Click on the cog in the upper right portion of the screen and then press View all Outlook settings.

Finally click on Rules and put in the name and logic of the rule you wish to make.