If the password for sharepoint farm account has been changed

Striving be more precise with what I am saying.

what you should do if you use an account as a service account and the password got expired?

 [SharePoint 2010]

A short version: Central Administration - > Security -> General Security -> Configure service accounts

A more deep version:

Opposed to 2007, to change the password\account for service account much easier in 2010:Managed Accounts in SharePoint 2010

My favorite author's sharing on this topic: SharePoint 2010 New Feature: Managed Accounts 

And an excellent article by Todd Knildt: Changing SharePoint 2010’s Service Account Passwords

[SharePoint 2007]

Here is the short version:

 Both the Web application pool and the Timer service store the credential information and will continue to function even after the password has expired. However, if the Web application pool or the Timer service is restarted, the stored credential information is lost, and the Web application pool or the Timer service cannot be started until the credential information is updated.

  To fix Timer service and central administration:

stsadm -o updatefarmcredentials -userlogin "Your Domain\Your User" -password "Your Password" 

 There is no equivalent user interface procedure for these procedures. You must use the command line to perform these password changes. Do not attempt to change the passwords using Windows Service Manager.

 To fix application

{Central Administration Url}/_admin/operations.aspx

Service accounts - 

Here is the long and more deep version

 

Make sure that you read this article - It has a useful script also.

How to change service accounts and service account passwords in SharePoint Server 2007 and in Windows SharePoint Services 3.0

 Another great source about services accounts How to change service accounts and their passwords in MOSS and WSS 3.0

 A quick summary:

Rule #1: Don't change account pool identities directly in IIS (unless it's the only way).

Rule #2: Make the changes while logged in as a domain administrator.

Here is on blogpost with short possible instructions to fix the bad password
and another one

 Best practices: what accounts are needed to install Sharepoint Farm correctly