One of the great things about Office365 is the constant updates, changes and improvements. All without you needing install, maintain or purchase anything.
So, bear in mind these instructions might change slightly though I doubt too much. In any event, you can always refer to the Office support site at Microsoft.
https://support.office.com/en-gb/
To change your password, log into the portal, access the settings from the gear icon and voila! There’s the change password link.
https://portal.office.com (you should be seeing this in your dreams by now!)
NOTE : bear in mind your Outlook, phone and anything else that accesses your Office365 account will probably need the password updating. With any luck, it’ll prompt you nicely and you can just enter the new one.