Depends on which model you have. To find the model identifier open System Profiler in the Utilities' folder. It's displayed in the panel on the right. You may also find the complete model information by selecting About This Mac from the Apple menu.
Note that you want to upgrade, not update. You have the last update for Sierra. You need to find out if you can upgrade to High Sierra, Mojave, or Catalina.
How to upgrade to macOS High Sierra
How to Upgrade to Mojave
How to upgrade to macOS Catalina
Open the App Store and look for macOS Catalina. Download the installer. If your model cannot run Catalina, then you will be so informed. The download is 4.9GBs.