Open Start Menu -> View Advanced System Settings -> Environment Variables -> System Variables

Click "New"
Variable Name : MAVEN_HOME
Variable Value: C:\apache-maven-3.6.0
Click "Ok"

Next Add PATH in Same System Variables.

Click "New"
Variable Name : Path
Add a new value by clicking 'New' button in top right corner.
Variable Value : %MAVEN_HOME%\bin
Click "Ok"

Then Open CMD, then run

mvn -version