and still couldn't figure out how to rename the message.

You can't rename a message

So, if you want to “rename” it, then you could delete the you don't like broadcast uses, then go out to backdrops(or back to sprite if ur in backdrops) and u see, the message that is unused is deleted, now make new - name it you want again and done. Hope this work!

but another thread said I could

Pikapika200 wrote:

but another thread said I could

the guy who said it probably didn't know or it was a old topic using scratch 2.0 (which maybe had rename broadcast but I don't know)

You can't rename it. Make a new broadcast…

You used to be able to in Scratch 2.0, but the ST hasn't put it back yet in 3.0, probably they forgot to. You can however replace all the existing instances of the message you want to rename with a new message.

While you can't rename it, a neat trick is to make a new broadcast, make sure there are no uses of the previous one, and then refresh the page. This should remove all unused broadcasts. Hope this helps.
@Lapras9998

whatever. the names have become inaccurate, but I don't care.