Right click on My Computer >> Properties >> Advanced system settings >>
System Properties window will get displayed
Under Advanced >> Environment Variables
Click on New to set Environment Variables
Variable name: JAVA_HOME
Variable value: C:\Program Files\Java\jdk1.8.0_121
Variable name: M2
Variable value: %M2_HOME%\bin
Variable name: M2_HOME
Variable value: C:\Program Files\Apache Software Foundation\apache-maven-3.5.0
Variable name: Path
Variable value: %M2_HOME%\bin
Then click on Ok, ok, ok.
Now restart you command prompt and check again with “mvn –version” to verify the mvn is running, you may restart your system also.
It should be working now.