You can upgrade PHP 8 on Windows by following these steps:

Download the latest version of PHP 8 for Windows from the official PHP website (https://windows.php.net/download/).
Extract the downloaded ZIP archive to a directory of your choice, such as "C:\php".
Rename the existing PHP directory to something else, such as "C:\php-previous".
Move the extracted PHP directory to the location where the old PHP directory was located, such as "C:\php".
Edit the system environment variable "PATH" to include the new PHP directory. To do this, right-click on "My Computer", select "Properties", then click on "Advanced System Settings". Click on the "Environment Variables" button, locate "Path" under "System Variables", and add the path to the new PHP directory at the end of the existing value. For example, if the old PHP directory was "C:\php-previous" and the new PHP directory is "C:\php", you would add ";C:\php" to the end of the existing value.
Restart any services that rely on PHP, such as Apache or IIS.

After completing these steps, you should be running the latest version of PHP 8 on your Windows system.

How to Install PHP 8