function Set-SPOContentType
{

param (
[Parameter(Mandatory=$true,Position=1)]
    [string]$Username,
[Parameter(Mandatory=$true,Position=2)]
    $AdminPassword,
[Parameter(Mandatory=$true,Position=3)]
    [string]$Url,
[Parameter(Mandatory=$true,Position=4)]
    [string]$ListTitle,
[Parameter(Mandatory=$true,Position=5)]
    [string]$ContentTypeName,
[Parameter(Mandatory=$true,Position=6)]
    [string]$CTFieldInternalName
    )

$ctx=New-Object Microsoft.SharePoint.Client.ClientContext($Url)
$ctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Username, $AdminPassword)
$ctx.ExecuteQuery()

$rootWeb = $ctx.Web
#$sites  = $rootWeb.Webs

$ctx.Load($rootWeb)
#$ctx.Load($sites)
$ctx.ExecuteQuery()

   #
   # -1- for each subsites in SC
   # foreach($site in $sites)
   # {
   # $ctx.Load($site)
   # $ctx.ExecuteQuery()
   #
    Write-Host ". Site name: "$rootWeb.Title -f Cyan # Write-Host $site.Title -f Cyan
    try
    {
    $ll=$rootWeb.Lists.GetByTitle($ListTitle)


        $ctx.Load($ll)
        $ctx.Load($ll.ContentTypes)
        $ctx.ExecuteQuery()
        Write-Host ".. List name: " $ll.Title
             foreach($cc in $ll.ContentTypes)
             {
            $ctx.Load($cc)
            $ctx.Load($cc.FieldLinks)
            $ctx.Load($cc.Fields)
            $ctx.ExecuteQuery()

                if($cc.Name -eq $ContentTypeName)
                {
                  Write-Host "... CT name: "$cc.Name -f Cyan
                  if($cc.ReadOnly -eq $true)
                  {
                  Write-Host  -f Cyan "... ." $cc.Name" CT - ReadOnly is"$cc.ReadOnly" ...setting to False"
                  $cc.ReadOnly =$false
                  $cc.Update($false)

                    foreach($field in $cc.FieldLinks)
                     {
                      if($field.Name -eq $CTFieldInternalName)
                        {
                        $ctx.Load($field)
                        $ctx.ExecuteQuery()
                        Write-Host  -f Yellow "... .. Field Name:" $field.Name "Required:" $field.Required "...setting to False"
                        $field.Required =$false
                        Write-Host  -f Green "... .. Required:" $field.Required
                        $cc.Update($false)
                        }
                     }
                  Write-Host  -f Cyan "... setting CT ReadOnly back to True"
                  $cc.ReadOnly =$true
                  $cc.Update($false)
                  $ctx.ExecuteQuery()
                  Write-Host "___"
                  }

                }

             }

        }
        catch
        {
        write-host "$($_.Exception.Message)" -foregroundcolor Yellow
        }
  }


# Paths to SDK. Please verify location on your computer.
# Download https://www.microsoft.com/en-us/download/details.aspx?id=42038
Add-Type -Path "c:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15ISAPIMicrosoft.SharePoint.Client.dll"
Add-Type -Path "c:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15ISAPIMicrosoft.SharePoint.Client.Runtime.dll"

# Insert the credentials and the name of the admin site
$Username="user@MS"
$AdminPassword=Read-Host -Prompt "Password" -AsSecureString
$AdminUrl="https://SITE.sharepoint.com"
$ListTitle="DocumentsTest"
$ContentTypeName="Document"
$CTFieldInternalName="WorkFax"


Set-SPOContentType -Username $Username -AdminPassword $AdminPassword -Url $AdminUrl -ListTitle $ListTitle -ContentTypeName $ContentTypeName -CTFieldInternalName $CTFieldInternalName