Turn all tracks down at once.

Click on any track, hit CTRL+A, then drag down any fader. All the faders will come down proportionately, so they retain their relative levels.

Depends on the complexity of the project routing, if there are submixes/folders and/or child fx sends that route outside folder/buss, compressors on folders etc. it's not going to be proportional. You can place a volume control 1st in line on the master FX chain if you are that far into the mix already.

Alternately you could add a gain plugin before any other effects to the master track and lower that.