Hi James,
Thank you for your interest in Syncfusion Products.
Could you please provide us more information on what do you like to validate when the 'Update' toolbar item is clicked? Meanwhile, you can also try implementing the validations in the "ClientOnItemSelect" client side event of the Toolbar Item. If the validation fails, you can stop the postback by just returning 'false'. You can find more information of using them in the Online Sample in the link below
http://samples.syncfusion.com/ASPNET/8.2.0.18/Web/Tools.Web/samples/3.5/MenuPackage/Toolbar-BasicFeatures/ClientSideEvent/cs/ClientSideEvent.aspx?args=2Kindly try the above and let us know if this works.
Regards,
Lakxman Kumar C