This can be done from AndroidManifest.xml line 22:
<activity android:name=".MainActivity"></activity>
<activity android:name=".WebViewActivity" />
<activity android:name=".ImageViewActivity" />
<activity
android:name=".SplashScreenActivity"
android:label="@string/app_name"
android:theme="@style/SplashTheme">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
Change to the following to make MainActivity the launcher:
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".WebViewActivity" />
<activity android:name=".ImageViewActivity" />
<activity
android:name=".SplashScreenActivity"
android:label="@string/app_name"
android:theme="@style/SplashTheme">
</activity>