Head to store.steampowered on your favorite macOS browser and hit on Install Steam in the top right 4️⃣ corner. Once you're taken to the download page, hit Install Steam again. Allow the download to go through. Once done, 4️⃣ click on the Downloads icon in the top right corner then double-click the steam.
Steam to Drop Support for 4️⃣ Older MacOS Versions, and With It 32-Bit Games. Steam will stop supporting macOS 10.13 (High Sierra) and 10.14 (Mojave) on 4️⃣ Feb.15, 2024. It's the end of an era, at least for Mac gamers.