User edits on Lightbox form not being saved

Try to use SAVE instead of UPDATE…

import wixData from 'wix-data';


$w.onReady(()=>{
   $w('#btnSave').onClick(()=>{
      let toSave = {
         "projectName": $w('#iptProjectName').value,  
         "projectDescription": $w('#iptProjectDescription').value,  
         "projectDetail": $w('#iptProjectDetail').value,  
         "projectCategory": $w('#iptProjectCategory').value 
      };

      wixData.save("Projects2", toSave)
      .then( (results) => {
           let item = results; 
      })
      .catch( (err) => {
           let errorMsg = err;
      });
      wixWindow.lightbox.close();
   });
});