How to change the minSdkVersion of a project?

In your app/build.gradle file, you can set the minSdkVersion inside defaultConfig.

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.3"

    defaultConfig {
        applicationId "com.name.app"
        minSdkVersion 19    // This over here
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }