question

@RaytheonXie-MSFT Thank you for your support, this would be a great help, if you provide me the documentation how can I update file 'modified by' and 'security classification' fields. using relative path.

Hi @NagavalliPuli-7205 ,
Per my research, The REST API does not currently support working with Managed Metadata or Taxonomy fields. If you look at the REST API reference you'll find no mention of SP.Taxonomy or anything under SP.Taxonomy
https://docs.microsoft.com/en-us/previous-versions/office/developer/sharepoint-rest-reference/jj860569(v=office.15)?redirectedfrom=MSDN

After trying to update 'modified by', I found that the rest api only works for list but not document library. Here is a similar issue with yours for reference.
https://docs.microsoft.com/en-us/answers/questions/293328/is-there-a-rest-api-to-modifyupdate-readonly-prope.html

As a workaround, you can update 'modified by' by powershell
https://www.sharepointdiary.com/2016/11/update-created-by-modified-by-created-at-modified-at-values-using-powershell.html

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Hi @NagavalliPuli-7205 ,
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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