How can I adjust the brightness of my webcam?

If not already installed, install the Video for Linux 2 Universal Control Panel (v4l2ucp), that will allow you to adjust your video settings

Install guvcview (still works on 20.04):

sudo apt install guvcview

On Image Control tab, there is a brightness slider control

Unfortunately the control panel, v4l2ucp, has been removed in 20.04 along with all other qt4 applications. The package is still available here, and it installs just fine as long as you have qt4 available from a third-party repo like the one in this answer.