Since Laravel Forge supports multiple versions of PHP, it’s nowadays rather a simple operation to upgrade PHP-version. This is my workflow which I got as a recommendation from the Laravel Forge support.
First, install the new PHP version. Go to your server and click PHP from the menu on the left. Scroll down on the page and click “Install” on the desired version.
The installation can take quite a while, so be patient.
When the new version is installed, visit the first site on your server and click “Meta” in the menu on the left. Change PHP-version to the newly installed version.
Check that the site works and the correct PHP version is loaded.
Rinse and repeat this for each site.
When all sites have been upgraded to the latest version, you can uninstall the old PHP version.