You have 3 ways to reset your AirVisual Pro:
A soft reset that will remove all your data. Go to the settings menu > 'About' > 'Pro reset' > 'Reset settings & clear data'. All your data will be wiped. This is a good option if you're planning to give/transfer your Pro to someone else and you don't want any logged data to be on the device. When you restart the Pro, you will be prompted with the original initialization wizard.
A soft reset that will remove your settings but keep your measurement data. Go to 'Settings' > 'About' > 'Pro reset' > 'Reset to factory settings'. All your settings will be wiped, but your historical measurement data will be kept.
A hard reboot that will get you back to the factory settings of the device (your historical data will not be affected!). Take a pin and insert it in the hole that is in the red square. You should hear a click.