SQL Server: How to properly use xml parameter for Update procedure

I am pretty new to XML and am looking for a way to use XML from an input parameter for the below part of a stored procedure (using SQL Server 2012).

The XML is submitted via JS / Ajax and looks like this:

var xmlMain = '<root><title>' + title + '</title><summary>' + summary + '</summary><post>' + post + '</post><departmentID>' + departmentID + '</departmentID></root>';

The parameter in SQL is defined as:

@xmlMain xml

To select from the XML the following works:

SELECT  [Xml_Tab].[Cols].value('(title)[1]', 'nvarchar(100)'),
        [Xml_Tab].[Cols].value('(summary)[1]', 'nvarchar(500)'),
        [Xml_Tab].[Cols].value('(post)[1]', 'nvarchar(max)'),
        [Xml_Tab].[Cols].value('(departmentID)[1]', 'int')
FROM    @xmlMain.nodes('/root') AS [Xml_Tab]([Cols])

What I can't figure out is how to apply something like this to the below:

UPDATE  RC_Posts
SET     title = @title,
        summary = @summary,
        post = @post,
        departmentID = @departmentID
WHERE   postID = @postID

I hope someone here can help me with this.

Many thanks in advance, Tim.