If this is a Windows host it will not update to V16. The procedure for Windows is to backup the PBX, take your backup out of the 3CX folder, and uninstall the system. Then download the fresh installer from our site and while configuring the new installation choose the option to Restore from Backup.
For Linux hosts, you you need to already be on Debian 9 (Stretch) and the PBX must already be on V15.5SP6 (15.5.15502.6) before you can upgrade to V16. If this machine's OS is lower than the above, it may be a good idea to take a backup of the machine, wipe the drive, and install fresh using our Debian ISO. And just like Windows, during installation you will have the option to restore from backup.
Note: 16.0.3 is a test version, if you update you will get the official stable release which is 16.0.2.910