Depending on your software you need to follow the guides below.
FreePBX Official Distro Install
Follow the guide called "Upgrading FreePBX with the Official FreePBX Distro" if you installed the Official FreePBX Distro
FreePBX Source or other Distro Install
If you installed FreePBX from Source or other Distros such as;
Follow the guide called "Upgrading standalone FreePBX GUI"
All PBXact systems require a Support Contract (Peace of Mind Package) to receive updates so make sure you have a active POMPs. Follow the guide called "Upgrading PBXact"