Вы можете выполнить следующие шаги:
1. Go to Preferences->Maven-Installation-> Click the "Add " button to add external maven you had downloaded ,Then click the "Ok" or "Apply' button to save.
2. For you current maven project, you need to right click the project or pom.xml file ,choose "Run As"->"Run Configurations", find current maven run setting, at the bottom there's a "maven Runtime" setting , for the dropdown list to select the previous added maven ,Then click the "Apply" button to make it affect .