Recently upgraded to WordPress 4.9.x and found that you cant update widgets, right? well you are not alone.
I also faced similar issue today, tried to found solution into forums and stuff but they suggested to activating/deactivating plug-ins and stuff. But it did not work.
After tweaking a while I found solution which worked for me.
Goto Screen Options on top-right corner and then Enable accessibility mode this will let you edit widgets.
Check screen-shot for better understanding.