This field is not intended to be changed.That being said you can change the label for it if you want to edit the core files.Open /include/client/open.inc.phpOn line 58 change "Help Topic" to whatever you want.On line 75 change "Select a Help Topic" to whatever you want.This change is only for clients (aka Users) and will not affect Staff side.This will break sites using language packs for this field.