This blog shows you SSMS Upgrade from 17.x to 17.3 without first uninstall the existing SSMS.
Previously to upgrade SSMS I need to manual the previous version of SSMS until this version, 17.3.
I was using 17,1, when I have seen the new version notification then I downloaded the SSMS version 17.3 from the following link.
Once you finish downloading then you will get the file SSMS-Setup-ENU-Upgrade.exe. Run the exe,
Click Install. You will see your installation is in progress.