How to Change Service Accounts in SharePoint 2013 using PowerShell

Change Service Accounts in SharePoint 2013
If you have created a new managed account or want to change the mapping of managed Account with SharePoint 2013 Services, Go to:

• Central Administration >> Security 
• Under the General Security section, click Configure Service accounts. 
• Pick the Appropriate Service and Service Account from the Drop downs
  
  
• Click "OK" to Save your changes

Read more: http://www.sharepointdiary.com/2014/09/configuring-managed-accounts-in-sharepoint-2013.html

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

#Custom Function to change Service account of given service
Function Set-ServiceIdentity($svc, $UserName)
{
    #Get the current service account
    $ProcessIdentity = $svc.Service.ProcessIdentity
    
    if ($ProcessIdentity.Username -ne $UserName) 
    {
       $ProcessIdentity.Username = $UserName
       $ProcessIdentity.Update()
       Write-Host "Service Account Set!"
    }
}

#Get the Service
$Service = Get-SPServiceInstance | Where {$_.TypeName -eq "Claims To Windows Token Service"}

#Call the function to Set "Local System" Identity to given service
Set-ServiceIdentity $Service "NT AUTHORITY\SYSTEM"

The above script changes the specified service's service account.

How to change the farm service account in SharePoint 2013:
We still rely on STSADM tool to update SharePoint 2013's Farm Account. Use this command line to update SharePoint Farm's service account credentials:
stsadm -o updatefarmcredentials -userlogin "DOMAIN\username" -password "Password here"

Change Service Application Pool's Service Account: 
To change application pool's service accounts, use this PowerShell script:

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

#Custom Function to change Service account of given service
Function Set-ServiceAppPoolIdentity($SvcAppPool, $UserName)
{
    if ($SvcAppPool.ProcessAccountName -ne $UserName) 
    {
       Set-SPServiceApplicationPool $SvcAppPool -Account $UserName
       Write-Host "Application Pool Service Account Updated!"
    }
}

#Get the Service
$SvcAppPool = Get-SPServiceApplicationPool | Where {$_.Name -eq "Service Application App Pool"}

#Call the function to Set "Local System" Identity to given service
Set-ServiceAppPoolIdentity $SvcAppPool "Crescent\SP2016Admin"

Change Web Application App Pool Accounts:

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

#Custom Function to change Service account of Web App Pool
Function Set-WebAppPoolIdentity($WebAppPool, $UserName)
{
    if ($WebAppPool.ManagedAccount.UserName -ne $UserName) 
    {
       #Get the Managed Account
       $ManagedAccount= Get-SPManagedAccount $UserName
       #Set the Managed Account
       $WebAppPool.ManagedAccount = $ManagedAccount
       $WebAppPool.Update()
       Write-Host "Web Application App Pool Account Updated!"
    }
}

#Get Web App's App Pool
$WebAppPool = (Get-SPWebApplication "http://intranet.crescent.com").ApplicationPool

#Call the function to Set the Managed Account to Web App Pool
Set-WebAppPoolIdentity $WebAppPool "Crescent\SPAdmin"