Output levels are adjusted on a preset basis. But if your input level is adjusted properly, you shouldn’t have to tweak this most of the time. I recommend using H9 Control to adjust output levels for your presets. But if you want to adjust it on the front panel:
Press and hold the Left Footswitch for 2 seconds and the display will show [OUTLVL], and then a second later  (or the current value).
Continue holding the Left Footswitch while turning the Knob to adjust output level from -12 dB [OUT-12] to +12 dB [OUT+12] in 1 dB increments.
Release the Footswitch – the Preset will return to its previous Active or Bypassed state.
The next time you do a Press and hold of the Left switch it will return to the last thing you adjusted.
Note: Preset output levels must be saved even if there are no other changes to the preset or the output level will revert to its previous setting the next time the Preset is loaded. In other words, if you want the adjusted output level to ‘stick’, you must re-save the Preset.