Unlike the system updates, Nintendo does not specify what improvements they have made to the controllers. But, whatever has been tinkered with, you will find that there are updates for both the left and right Joy-Con.
Remember that you can only update controllers that are connected to the console, for those with more than the two Joy-Con that come with the portable home console.
How To Update The Nintendo Switch Joy-Con Controllers
1. Select the System Settings on the HOME Menu.
2. Scroll down through the System Settings menu and select Controllers and Sensors.
3. Select Update Controllers, to start the controller update process. Please note that this will only update any Joy-Con controllers that are connected to the Nintendo Switch console.