To remove the trial error from existing web app or site, all you need to do is to update the Telerik.Web.UI.dll with the registered one as shown in the following online resources:
Upgrading RadControls Trial to RadControls Developer license or newer version
Getting tired of seeing trial messages in your ASP.NET AJAX application?
Upgrade the Version of Your Telerik ASP.NET Controls in 6 Easy Steps
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