SCCM 1802 is the latest baseline version used for clean installs.
SCCM 1806 is an update to 1802 and the most effective way to install is using an SCP and the update is presented in the console.
If you are impatient for the latest release and it is not in the console you can hurry it up using the EnableFastUpdateRingXXXX.ps1 script that Microsoft provide – where XXXX is the new version number. The script uses the site server name as its only parameter, e.g. .\EnableFastUpdateRing1806.ps1 CM01 where CM01 is the site server name.
More info here:
Check the ADK version and if need to update do this before the SCCM in-console update. Remove the existing ADK and then install the newer version. More info here:
Also a good idea to temporarily stop any site maintenance tasks while running the update.
Full pre-install checklist here:
Suggest you run the prereq check before starting the install and address any highlighted issues.
Thanks to Mr Brady and the System Center Dudes for their continued excellent contributions to the SCCM community.