There's no doc handy for this... the PW change should be a straight forward operation of changing it in AD and then on the services that account is associated with in the SCM.
Based on your other post, however, it sounds like what you want is a procedure for changing the actual account, not just the PW on the standing account which is a bit of a different beast.
The permissions wizard will be updated soon to help you select new accounts for the directory facing and messaging facing accounts for Unity and allow you to change them on a running system - and make sure they have all the juice they need to. That will be in Unity 4.0 however...
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.