SQL Server: How to use Update statement with xml input

I use SQL Server 2008 and have a stored procedure with one input parameter formatted as xml. This xml is a list of names, one word each without spaces.

For each of these names I want to check if they already exist in a table, if no then they should get added to the table, if yes then they should get updated there.

So far I have the part to add them if they don't exist yet which works as intended but I can't figure out how to realise the updating part.

Can someone here help me with this ?

Just for demonstration this would be the update part as a stand-alone (if I would have one input instead of the xml):

UPDATE  RC_PermissionsUsers
SET     ntid = @ntid,
        departmentID = @departmentID,
        role = @role
WHERE   ntid = @ntid

The rest of my procedure for the insert part (working):

BEGIN

    SET NOCOUNT ON; 

    BEGIN TRANSACTION;

        BEGIN       
            INSERT INTO RC_PermissionsUsers
            (
                            ntid,
                            departmentID,
                            [role]
            )
            SELECT          ParamValues.ntid.value('.', 'varchar(255)'),
                            @departmentID,
                            @role
            FROM            @xmlUsers.nodes('/users/ntid') AS ParamValues(ntid)
            WHERE NOT EXISTS 
            (
                SELECT      ntid
                FROM        RC_PermissionsUsers
                WHERE       ntid = ParamValues.ntid.value('.', 'varchar(255)')
            )               

        END

    COMMIT TRANSACTION;  
END

Many thanks in advance, Tim.