heres what may happen, speaking from experience with iOS. an update should not erase anything, but if something happens during said update, you may have to restore the phone, losing everything. that could happen a number of different ways: the phone powering off during the update, the phone simply freezing halfway through. im not sure how often this happens with Android, but I know with iOS it was pretty common. dont do the update unless your battery has a pretty good charge and just let it do its thing