$loadInfo1 = [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client")
$loadInfo2 = [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.Runtime")
$webUrl = Read-Host -Prompt "HTTPS URL for the SP Online 2013 site"
$username = Read-Host -Prompt "Email address login"
$password = Read-Host -Prompt "Password for $username" -AsSecureString


$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($webUrl)
$ctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password)
$web = $ctx.Web
$lists = $web.Lists
$ctx.Load($lists)
$ctx.ExecuteQuery()

$lists| select -Property Title

$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($webUrl)
$ctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $securedPassword)

$field = $ctx.Site.RootWeb.Fields.GetByInternalNameOrTitle($fieldTitle)
$field.Description = $fieldDesc
$field.UpdateAndPushChanges($True)
$ctx.ExecuteQuery()