You will automatically get upgraded to the new look soon.
If you don't want to wait, youcan switch to it today by logging into Gmail and clicking Basculez vers le nouveau look.

Hate Gmail’s new look? Here’s how to get the old look back.
You can switch to it today bylogging into Gmail and clicking l'icône options et sélectionnez revenir à l'ancienne apparence temporairement

Gmail nouveau look plaintes : ...