I don't see why まで is what would change here. Like if you said "go up to the 3rd floor" and then contrasted that to "go down to the 3rd floor," the thing that changed is what verb you used. のぼる versus おりる.
まで just marks the limit of the action.
まで just means 'to' doesn't it? Like 駅まで歩きます means [I] 'walk to the station'. The opposite of まで would be
から (from).
Hey Victoria, a common mistake beginner language learners make is that they learn words in isolation without looking at how they are used in a sentence. From your question, I suspect that's what you did too. I made this mistake when I started learning and it led to a lot of problems down the road, so I hope you take this piece of advice. Jisho has an excellent database of example sentences which I hope you will be able to take advantage of :)
what's the opposite of まで？
If まで means "up to" as in "go up to the 3rd floor" then what is "down to"?