Go to Advanced -> Upgrade. You will see "Source Code" at the top, that's how you upgrade version of your FusionPBX. Click on the commit id to see what has changed over your version up now. Don't forget to tick all checkboxes before you hit the execute button. Also you should take a backup before you upgrade.
Official latest release right now is 5.0.1 which is released exactly one year before now. But I am using version 5.0.10 which I upgraded to, the way I told you. For downgrade, I don't think there is an official way other than taking a backup and restoring it back.
