The preposition "nach" can mean several things, depending on context. Here it indicates direction towards a place. Thus "nach Portugal" means "to Portugal", as in "Ich reise oft nach Portugal," "I often travel to Portugal". So, if "Komm nach oben" means "Come upstairs", how would you say "Come downstairs" in German?
And here is the answer to yesterday's question: "Cats like to be on top" (or, if you like, upstairs).
Depending on context, "oben" can mean upstairs, above or on top, while "unten" can mean downstairs, below or on the bottom. So ... what does the following mean in English? "Katzen sind gern oben."