Take a full backup of your current FreePBX installation. Install a new instance of FreePBX and restore from your full backup. After the installation is complete you should be able to register your new installation to your current Deployment ID with Sangoma which should tie all existing licenses to this new installation.
If you can I would possibly delay this upgrade for another little while as FreePBX17 will be released some time this year and there is no direct upgrade path from 16 to 17 (you’ll have to do the same process as above to move from 16 to 17).