You should be able to modify the password through the services msc beforehand. Do not restart any services until the password(s) have been changed via the services msc on all nodes of any given cluster. Once you have completed this you can then stop/start each instance and optionally test failovers.

Normally using the services msc is not the recommended tool since special directory and registry permissions are set by the SQL Configuration Manager when it is used. However since it is a password change only this is not an issue.

Just remember that if you change the actual service account you do need to use the SQL Configuration manager.