Go to https://login.microsoftonline.com and login with your Microsoft 365 account.
Click on the Settings icon at the top right then click on the "Change your password ->" link (see below) and follow instructions.
Once it is changed that will affect not only your other Microsoft 365 Apps but your school Apple ID password. So you will likely be prompted to enter the new one at some point.
Please email Tech Help if you have questions or having technical issues changing your password.