NOTE: This will NOT change the password to the app, just what classlink has saved.
Navigate to Classlink and sign in
Select your profile icon in the top right and click "Settings"
Click the "App Passwords" tab
Scroll down to the app you need (usually either Qmlativ or IEPOnline)
Click the pencil icon to edit
Either:
Select the red 'X' to remove the app password entirely. This will cause you to be prompted to enter the password when you next open the app
Select the pencil icon again to edit your password that is saved. Be sure to hit save once you have entered your new password.