To change your Outlook 365 password, you will need to login to your Office 365 account via the web application.
Once logged in you will be prompted with the following screen, select the Icon that says Outlook.
Selecting Outlook the next screen that appears will be your email inbox.
The next step is to click the cog wheel icon at the top right of your inbox screen which will expand a drop down list, select Outlook 365 settings.
This will take you to the next screen listing a few settings options, select the Password option to proceed to the next step.
You will be prompted to enter your Old password and to Create new password.