You can find all your purchased apps in the "Purchased" section of App Store.

  1. Tap App Store.
  2. On an iPhone or iPod touch: Tap Updates, then tap Purchased.
    On an iPad: Tap Purchased.
  3. Scroll to find the app that you want to download.
  4. Tap the Download icon.

After the app downloads, you can open it from your Home screen.

Previously purchased apps which is no longer sold in the App Store can re-downloaded from purchase history, please visit this Apple's support page to learn more.