Manually update Visual Studio 2017

Tools -> Extensions and Updates -> Updates

You can update it from Notifications Hub left side of Visual studio.

In Notification Hub/Pane click on "Visual Studio Update" is available.

Or You can also update it from Visual Studio Installer.

https://docs.microsoft.com/en-us/visualstudio/install/update-visual-studio

Launch the visual studio installer (hit the windows key and type the name, you'll find it). It will then ask you to allow it to update itself, after which you will be offered the update.

To update the core , let say from 15.4 > 15.9 , you need launch Visual Studio then Help > Check for updates

To add\remove\update other sub component , you will need to run the Visual Studio installer and pick the require option.