Let's go back downstairs.

let's verb

— Used to form the hortative of verbs, equivalent of the first-person plural imperative in some other languages. — let us; forming first-person plural imperative

go Verb

— To move; to move away from a place; to move farther from the person who is talking; to move from one place to another place. I go to the seashore every summer. Kathy goes to the seashore every summer, too. I went to the seashore last year. She's gone to the seashore. I'm going to the seashore next year.

back Preposition

— Toward the rear. He went back behind the stands. — To a place again. I didn't like it, so I sent it back. He went back to the same house. They came back again.

back Noun

— The back is the rear part of something; it is the part in the other direction from the front. I went to the back of the house. — The back is the rear part of the human body. He had a scar on his back.

back Verb

— If you back something, you support it. The Republicans backed the bill. — If you back up, you move backward. He put the car in gear and backed right into the garage door.

downstairs Preposition

— If something is downstairs, it is on the level at the bottom of the stairs. Let us head downstairs, from the bedroom to the kitchen below. The downstairs bathroom is always busy.