Change the window title

The default window that Bevy spawns is created by the WindowPlugin.

When you add your DefaultPlugins you can customize the settings of each plugin with the DefaultPlugins::set method:

use bevy::prelude::*;

fn main() {
  let app_window = Some(Window {
    title: "Some title other than \"Bevy App\"".into(),
    ..default()
  });

  App::new()
    .add_plugins(DefaultPlugins.set(WindowPlugin {
      primary_window: app_window,
      ..default()
    }))
    // .. the rest of your app
    .run();
}

The Window component encapsulates all the settings related to how Bevy spawns your window across operating systems using the winit crate.

Typically in my projects I create a plugin specifically for the window with these settings:

use bevy::prelude::*;

const BACKGROUND_COLOR: Color = Color::srgb(0.4, 0.4, 0.4);

pub(super) fn plugin(app: &mut App) {
  let primary_window = Window {
    title: "Bevy game".into(),
    resizable: false,
    resolution: (800., 600.).into(),
    canvas: Some("#bevy".to_owned()),
    desired_maximum_frame_latency: core::num::NonZero::new(1u32),
    ..default()
  };

  app.insert_resource(ClearColor(BACKGROUND_COLOR))
    .add_plugins(DefaultPlugins.set(WindowPlugin {
      primary_window: Some(primary_window),
      ..default()
    }));
}

You can see more at my bevy-starter.