User blog comment:BrianBerta/Greeting Messages/@comment--20150807150246/@comment--20150807165029

Well once you edit it, it will be in effect for the new users who join your Wiki. It doesn't rewrite messages for people who have already had a welcoming message. So if you edited one of welcome messages, then everyone who is "new" makes an edit will receive that updated welcoming message. But it won't overwrite the people who already "received" on before you changed it.

And there are four welcoming messages to choose from as DEmerson listed here.

If you're using the talk pages, you click the "talk page" tabber and those four MediaWiki pages will tell you where to go to edit it. Same for the message wall. You may have to "create" the page on your Wiki if you can't find it in search. So just go to Special:CreatePage on your Wiki and type in the full "MediaWiki" page name you want to edit your welcoming messages with.