Scrip to update Sharepoint Online content type

Question

I have a content type "TestContentType" with three columns:
Document Type
Category
Sub-category
This content type is used on 18 document libraries, in 3 site collections.
The Category column needs to have a "description" added which currently has no description. Is there a way to achieve this via a script so I don't have to make change manually via the UI.
So far I have come up with the following script, but this so far only lists all list and document libraries. What I actually need is to be able to get the content type in this case "TestContentType" and then update the "Category" column
to include a description:
$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
Any suggestions would be appreciated.

Answer 1

This isn't the best way to do it, which would be manipulating SharePoint through the CSOM, however you can automate IE so that it runs through a series of actions.
The nice way would be to use something like the client-side code here:
https://sharepointpowershell.codeplex.com/
 I don't have the time to dig into it at the moment but i'd try to merge that CSOM approach with something like this script:
http://get-spscripts.com/2010/11/add-and-remove-site-content-types-from.html which uses the on-prem version.