The update is available only for Twitter 7.0 (Beta) through a server-side change. So, it might take a few hours for beta testers to get the new twitter UI.
With this new update, icons are more rounded, increased font size, there's a new retweet icon, profile pictures are circles now, the header photo is gone from the menu, it also rearranges the options in the menu. The new UI looks pretty good, you can easily opt in for twitter Beta by following the below steps.
- Join this Google Group.
- Visit this link and click "Become a Tester" to opt into the program.
- Click "Download Twitter from the Play Store" to update your app