Source hehe it's a system update , it won't erase your data , why would Microsoft delete your personal data with update , eg , I updated my lumia 920, 3 times , first the protico update , second gdr2 /amber update and lastly recent gdr3/black update , it didn't affected any of my personal data including wifi settings. If you flash your phone , yes then you will lose all your data including wifi settings and passwords.