Canon Speedlite 600EX II-RT Manual (en)

You can revert the settings of the Speedlite shooting functions and 
wireless shooting settings to their defaults.
Clear the settings.
Press function buttons 2 and 3 for 
seconds until <w> is displayed.
The Speedlite settings are cleared, 
and normal shooting and <a> 
flash mode will be set.
Clearing Speedlite Settings
Even when the settings have been cleared, the transmission channel and 
wireless radio ID for wireless shooting as well as the settings of the Custom 
Functions (C.Fn) and Personal Functions (P.Fn) will not be cleared.