Make a Flat Background light from the toolbar and put it at the top of your lighting design. Change it's Blend mode to Multiply. Now you can change the brightness of this light to change the brightness of the whole HDRI map. A brightness of 100 for the Flat Background is 100%, 50 is 50% and 200 is 200%. This is a great way to tweak the brightness of the overall lighting design without needing to adjust each light individually.
But please note, this will not affect the brightness of lights that are 3D Area Lights not on the HDRI map.