Is it possible to create a chat command which changes a Billboard GUI's text?

I've been trying but I either don't know enough about lua or it's not possible.

You can. For Example:

local TextLabel = --Your Billboard GUI Text

game.Players.PlayerAdded:Connect(function(Player)
	Player.Chatted:Connect(function(Chat)
		if Chat == "/e DIE" --[[Your Command]] then
			TextLabel.Text = "lol help"
		end
	end)
end)

That script allows everyone to change the text to "lol help" if they type "/e DIE"