While changing the PHP version for a domain in GoPanel is most commonly done through the GoPanel MultiPHP Manager interface, you can 
do it via the command line if you have SSH access to your server (typically as root.

Here's how you can do it using the "multiphp" command (which requires root access):
$ /go/multiphp gopanel.org php84

To change the PHP version to 8.4 for all domains in GoPanel
$ /go/multiphp all php84