How to change the gravatar image from front end?

They can change the image in their Gravatar account.

Or set a default from the backend which isn’t what you asked originally

add_filter( 'avatar_defaults' , 'default_avatar' );

function default_avatar( $avatar_defaults ){

$new_avatar = get_stylesheet_directory_uri() . '/images/default-gravatar.png';

$avatar_defaults[$new_avatar] = "Gravatar";

return $avatar_defaults;
}

Source Replace default avatar