To change your password, first off log into the main webmail interface as you would normally.

Then go to the drop-down menu at the top right-hand side (you should see your name)

Depending on whether you're using the “traditional” or “modern” web interfaces these both look slightly different (see screenshots below)

Change Password Dropdown

Select the “Change Password” option and a pop-up may appear warning you that you will be signed out of your accounts when you change your password.  Accept this warning and proceed to the next page which is the Change Password form.