Hello,
1) Best Sellers are products which are sold most frequently. It means the most popular products. They are determined automatically. You cannot set it manually.
2) To change the text of the hot badge you need to use loco translate WordPress plugin and choose the correct path.
You can watch the video in our detailed documentation on how to translate strings using loco translate.
https://www.youtube.com/watch?v=D3NsDdMzsls
3) You can change the shape from Theme settings >> Shop >> Product Labels.
Screenshot for Clarification: https://ibb.co/n3JS0fx
Best Regards.