You can find all your purchased apps in the "Purchased" section of App Store.
- Tap App Store.
- On an iPhone or iPod touch: Tap Updates, then tap Purchased.
On an iPad: Tap Purchased.
- Scroll to find the app that you want to download.
- 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.