Update site-design-powershell.md

Updated details on Set-SPOSiteDesign update cmdlet

Author: seansquires

docs/declarative-customization/site-design-powershell.md

@@ -33,8 +33,9 @@ The following cmdlets are available for managing site designs and site scripts f
 - **Remove-SPOSiteDesign**
 - **Remove-SPOSiteScript**
 - **Revoke-SPOSiteDesignRights**
-<!--
 - **Set-SPOSiteDesign**
+
+<!--
 - **Set-SPOSiteScript**
 -->
 
@@ -80,7 +81,7 @@ C:\> Add-SPOSiteDesign `
   -PreviewImageAltText "site preview"
 ```
 
-## **Add-SPOSiteScript**
+## Add-SPOSiteScript
 
 Uploads a new site script for use either directly or in a site design.
 
@@ -309,6 +310,47 @@ Updates a previously uploaded site design.
 Updates a previously uploaded site script.
 -->
 
+## Set-SPOSiteDesign
+
+Updates a previously uploaded site design. 
+
+```powershell
+Set-SPOSiteDesign
+  -Identity <SPOSiteDesignPipeBind[]>
+  [-Title <string>]
+  [-WebTemplate <string>]
+  [-SiteScripts <SPOSiteScriptPipeBind[]>]
+  [-Description <string>]
+  [-PreviewImageUrl <string>]
+  [-PreviewImageAltText <string>]
+  [-IsDefault]
+  [<CommonParameters>]
+```
+
+### Parameters
+
+|Parameter  | Description  |
+|-----------|--------------|
+|-Title                 | The display name of the site design. |
+|-WebTemplate           | Identifies which base template to add the design to. Use the value **64** for the Team site template, and the value **68** for the Communication site template. |
+|-SiteScripts           | An array of one or more site scripts. Each is identified by an ID. The scripts will run in the order listed. |
+|[-Description]         | The display description of site design. |
+|[-PreviewImageUrl]     | The URL of a preview image. If none is specified SharePoint will use a generic image. |
+|[-PreviewImageAltText] | The alt text description of the image for accessibility. |
+|[-IsDefault]           | A switch that if provided, applies the site design to the default site template. For more information see [Customize a default site design](customize-default-site-design.md) |
+
+The following example updates a previously created site design.
+
+```powershell
+C:\> Set-SPOSiteDesign `
+  -Title "Contoso customer tracking - version 2" `
+  -WebTemplate "68" `
+  -Description "Updated site design for list schema that tracks key customer data in a list" `
+  -PreviewImageUrl "https://contoso.sharepoint.com/SiteAssets/site-preview.png" `
+  -PreviewImageAltText "site preview - version 2"
+```
+
+
 ## See also
 
 - [JSON schema reference](site-design-json-schema.md)