Hi @Nagavalli Puli ,
For modified by field, you can refer to the following powershell script
#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
##Variables for Processing
$SiteUrl = "https://crescent.sharepoint.com/sites/sales/"
$ListName= "Projects"
$ID=6
$UserID="Salaudeen@crescent.com"
$TimeStamp = "2015/12/01 02:10:00 AM"
#Get Credentials to connect
$Cred = Get-Credential
$Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.Username, $Cred.Password)
#Setup the context
$Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteUrl)
$Ctx.Credentials = $Credentials
#Get the User
$User = $Ctx.Web.EnsureUser($UserID)
$ctx.Load($user)
#Get the list Item
$List=$Ctx.Web.Lists.GetByTitle($ListName)
$ListItem = $List.GetItemById($ID)
$Ctx.Load($ListItem)
#update created by column sharepoint online powershell
$ListItem["Author"] = $User
#Update Modified By
$ListItem['Editor'] = $User
#Set Created on & Modified on Time values
$ListItem["Created"] = $TimeStamp
$ListItem["Modified"] = $TimeStamp
#Update List item
$ListItem.Update()
$ctx.ExecuteQuery()
Write-host "Metadata values updated Successfully!" -f Green
For SP.Taxonomy, as far as I know, there is no such function to update Taxonomy by rest api after doing some research.
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