Neilfei That is a string used by localization.
Meaning: it is defined in a locale file.
See http://flarum.org/docs/extend/internationalize/ for more info ?
You will have to either create an extension to change the string or duplicate English language pack and change the string there.
Sorry if my answer isn't very explanative, but I hope it helps.
@Dominion can help you with internalization, just mention them.
If you want to do it by JavaScript, I can't help you now ?