Davide,
What you can do is go to Preferences > Install / Update > Available Software Sites and add the Photon update site: http://download.eclipse.org/releases/photon
. If you now check for updates, you should be offered the Photon update.
Note that other plugins you have installed (besides CodeMix) as well as the base Eclipse distribution you are upgrading might affect your ability to update, but the wizard should guide you through these options. If you want to be extra careful, you may take a backup of your Eclipse installation before applying the update.
An alternative would be to install CodeMix into a fresh Photon install – CodeMix extensions and User settings are stored in a common area, and if you use the same workspace (you should), workspace settings will be retained as well. Not much reconfiguration should be required at all.
Hope this helps.