You are using Linux Mint 18.3 "Sylvia" as your operating system and you are considering upgrading to the latest version Mint 19 “Tara”; Today you can do so as the upgrade path is officially open.
Yes, the Mint development team announced today that the upgrade path from the Mint 18.3 "Sylvia" operating system is now open to all three versions of Cinnamon, MATE, and Xfce, except for the no longer supported KDE version.
However, the upgrade is only recommended for those who want to explore the new features of Linux Mint 19 "Tara" as the Linux Mint 18.3 "Sylvia" version will be supported until 2021.
Upgrading is quite simple but before attempting to upgrade your system, make sure you have a backup of your most valuable files. Then open a terminal and install the mintupgrade package with the command "apt install mintupgrade"
After installing mintupgrade, you can upgrade Linux Mint 19 "Tara" by running the "download mintupgrade" command on your terminal.
Then the "mintupgrade upgrade" command will upgrade your system to the latest version. Once you are done you will need to restart your computer.
For more details, read them official upgrading instructions.