I have solved this issue yesterday.
Follow the steps :
Help -> Install New Software -> Add -> (Give any name suppose NewUpdate) -> add this url : https://dl-ssl.google.com/eclipse/plugin/4.2 -> OK
Now it will list the available updates, which should ideally be adt 20.x.x
Select the list items Let it be installed. Eclipse will restart and Its done.
I hope this will helpful for you 🙂