Qwant is set as the default search engine on Brave. However, if you have changed your search engine in the meantime and want to use Qwant, here is the solution.
Go to the main menu at the top right of your browser, then click on “Settings” at the bottom of the menu
Click on “Search engine”, there you will find a section of the same name in which you can select Qwant with the drop-down menu “Search engine used in the address bar”.
Your modification is automatically taken into account, Qwant is now your default search engine.