using CMS.CMSHelper;

using CMS.SettingsProvider;

using CMS.SiteProvider;

using CMS.DataEngine;

...

        string customTableClassName = "customtable.SampleTable";

        // Get data class using custom table name

        DataClassInfo customTableClassInfo = DataClassInfoProvider.GetDataClass(customTableClassName);

        if (customTableClassInfo == null)

        {

            throw new Exception("Given custom table does not exist.");

        }

      // Initialize custom table item provider with current user info and general connection

      CustomTableItemProvider ctiProvider = new CustomTableItemProvider(CMSContext.CurrentUser, ConnectionHelper.GetConnection());

        // Provide ID of item you want to edit

        int itemId = 1;

        // Get custom table item with given ID

        CustomTableItem item = ctiProvider.GetItem(itemId, customTableClassInfo.ClassName);

        // Set value of the custom table item field

        item.SetValue("ItemText", "Sample item");

        // Update item

        item.Update();