Shut down, then turn on while holding the Option (Alt) key. Select the USB drive, then click "Install macOS".
Apple no longer lists High Sierra in the App Store “Purchased” tab for most users. However, they maintain a legacy support page that links directly to the official installer.
macOS High Sierra 10.13.6 is a popular operating system for Mac computers, released on September 26, 2017. It's known for its stability, security, and performance. This version of macOS comes with several features, including: