Select all of your photos, make sure your active photo is the exposure you want, and then go to Setting > Match Total Exposures (or type Alt+Shift+Command+M). This will intelligently and automatically alter the exposures on all of the photos so they match the active photo.
Source:Â Lightroom Tip: How to Match Different Photo Exposures in One Click
This is a fantastic tip, especially if you shoot in Aperture priority mode.