It appears that many people are asking the same question. I found a few people who said in June they received this update. it appears that Lenovo and Android are sending them out in batches. A few users got the first updates and other didn't receive it as of yet.
I see on another forum that if you don't see the update you'll need to go to your setting and check it. There are times that the update won't notify you that it is available. You can also go to check for updates on your device. This will let you know if your area or where you live has received the new updates.
However, they are updating the 8.1 and now they have found a few bugs in this and worked on the 8.1.1 updates. Few people are starting to receive this update right now.
Your best option is to check for the updates for your device if you haven't yet received the automatic update or the notification.