Your SQL command is not valid: If I rip out most of it so it;s more readable then it comes down to:
Update Organisatie FROM Klanten WHERE [IDklanten] = @KlantID
Which is the format for a SELECT command not an update.
You need:
UPDATE Klanten SET Organisatie=@ORG WHERE [IDklanten] = @KlantID
And to supply the "@ORG" value via a parameter in the same way you do @KlantID.
Then a command.ExecuteNonQuery will do the update.