Here’s how you carry a bike up and down stairs:
Grab the rear brake level and wheel the bike up onto its rear wheel, so the handlebars are in front you at shoulder height. Grab the stem with one hand and around top tube near the seatpost with the other hand. Now lift the bike up and put the top of the saddle against your belly button. Now you can walk up and down stairs without having to awkwardly carry the bike at your side, and your core will support a lot of the bike’s weight as it pushes into your stomach. This is especially effective on narrow stairways with a turn or two.