A clean upgrade is best, so avoid the upgrade option
2. The best way to go from the older Mint 17 series to the newer Mint 18 series, is to apply a clean upgrade. Which means: format the hard disk partition on which the older version resides, and install the later version cleanly on that empty partition.
This reduces pollution, diminishes the risk of complications and is undoubtedly the fastest way to do it.
Only upgrading within a particular Mint series is safe and easy. Within a series, you can simply upgrade the older version to the later one by selecting the upgrade option in Update Manager...
The reason for this distinction is, that Linux Mint uses only LTS versions of Ubuntu as codebase. Within a Mint series that's always the same Ubuntu version, so the upgrade to a later Mint within the same series is technically no big deal. Most of the codebase remains the same then.
The Linux Mint 17 series contains 17, 17.1, 17.2 and 17.3. They all have the same base, namely Ubuntu 14.04 LTS. Which makes upgrading within that series swift and safe.
But the Mint 18 series (which starts with Mint 18 Sarah) is based on an entirely new Ubuntu, namely Ubuntu 16.04 LTS. This is a major leap, which increases the upgrade risks considerably.