Problem logging into localhost

[Tomorrow]

So, I don't have good internet connection when I am at my house, and I want to play, problem is, when I want to play localhost and I try to connect it says I can't connect because for obvious reasons I am updated to the new launcher. I want to know if there is a way around this? If so could I please have a step-by-step of it, or is there a way to downgrade back to the old launcher so I can play?