If you want to remain the Illustrator CC-CC2017 which is installed in your personal computer when updating to Illustrator CC2018, update with the following procedure. (Also FineCut/RasterLinkTools which is installed in your personal computer will be remained.)
Click [Update All] or [Update] button at right side of Illustrator CC.
Click [Advanced Options] button.
Switch off [Remove old versions] of [Advanced Options] checkbox, and click [Update] button.