(1) Start Android Studio and load your project.
(2) Go to gradle-wrapper.properties file and change the distribution URL to the version you want.
All available versions can be found in the link below: https://services.gradle.org/distributions/
(3) Go to your app-level build.gradle file and change the Android gradle plugin version.
All available versions and their matching Gradle requirement can be found int he link below: https://developer.android.com/studio/releases/gradle-plugin.html
(4) Finally, choose to "Sync Gradle" and that is all.