iOS
Settings > General > Software Update > Download and Install.
More info: https://support.apple.com/en-us/HT204204
Android
Settings > About device > System Updates > Check for Updates > Update to download and install the latest Android version.
macOS
Choose System Preferences > Click Software Update to check for updates > If any updates are available, click the Update Now button to install them.
More info: https://support.apple.com/en-us/HT201541
Windows
Select the Start button > Go to Settings > Update & Security > Windows Update.