How to change all the activity transitions at once in Android application?

You want to first create a <style> in res/styles.xml, like this:

    <style name="YourAnimation.Activity" parent="@android:style/Animation.Activity"> 
       <item name="android:windowEnterAnimation">@anim/your_in_down</item>
       <item name="android:windowExitAnimation">@anim/your_out_down</item>
    </style>

Then you can apply the style to a theme, in the same file:

    <style name="YourTheme" parent="android:Theme.Translucent">
       ...
       <item name="android:windowAnimationStyle">@style/YourAnimation.Activity</item>
    </style>

And finally apply the theme to your activities in the manifest:

    <activity
        android:name=".YourActivity"
        android:theme="@style/YourTheme" />

Look at these links for reference:

• Android Reference – Apply A Theme
• Android Reference – WindowEnterAnimation