How to change app:elevation programmatically

Try with using the following code

ViewCompat.setElevation(View, float)

Following the below link. Here showed how to make elevation on pre-lollipop device

Android AppCompat 21 Elevation

The accepted answer won't actually work. The answer should have been updated.

If you look at why mAppBarLayout.setTargetElevation(0) is deprecated. it states that:

@deprecated target elevation is now deprecated. AppBarLayout's elevation is now controlled via a {@link android.animation.StateListAnimator}. This method now always returns 0.

So to programmatically set the appbar layout you can only do it by creating a StateListAnimator such as appbar_state_unelevated_animator at the animator res folder:

<selector xmlns:android="http://schemas.android.com/apk/res/android">

<item>
    <objectAnimator
        android:propertyName="elevation"
        android:valueTo="4dp"
        android:valueType="floatType"
        android:duration="100"/>
</item>

then loading it in code like so:

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
        appbar.stateListAnimator = AnimatorInflater.loadStateListAnimator(applicationContext, R.animator.appbar_state_unelevated_animator)
    }

If I'm not mistaken you could also create a StateListAnimator by code but don't take my word for it.

Update:

In my experience when you set the app:elevation="//Any DP value//"

The Shadow of the app bar will be lost. Simple remove that line to get the appbar shadow again.

I would like to provide updated and full answer:

    switch (state) {
        case On:
            appBarLayout.setStateListAnimator(AnimatorInflater.loadStateListAnimator(getContext(), R.animator.appbar_elevation_on));
            break;
        case Off:
            appBarLayout.setStateListAnimator(AnimatorInflater.loadStateListAnimator(getContext(), R.animator.appbar_elevation_off));
            break;
    }

appbar_elevation_on.xml under RES\animator

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:ignore="PrivateResource">
    <item>
        <objectAnimator
            android:propertyName="elevation"
            android:valueTo="@dimen/design_appbar_elevation"
            android:valueType="floatType" />
    </item>
</selector>

appbar_elevation_off.xml under RES\animator

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:ignore="PrivateResource">
    <item>
        <objectAnimator
            android:propertyName="elevation"
            android:valueTo="0dp"
            android:valueType="floatType" />
    </item>
</selector>