How to change your password in Office 365
Login to the office 365 portal at https://portal.office.com
Click the cog in the top right hand corner, then Office 365 Settings
Select the ‘Security & Privacy’ option from the left-hand menu and then ‘Password’
Enter your existing password and new password twice and press submit to confirm.
You will now need to update this password on any computer or mobile device you use to access your Office 365 Mailbox.