Once you purchase an iPhone application with your iTunes account, you will never have to pay for it again. Even if you remove the app from your cell phone, a copy is available in iTunes as a backup. Just select the application for installation and sync your device with iTunes to put the app back on your iPhone. Once the app is back on your phone, the App Store will check to see if updates are available.
Attach your iPhone to your computer via the dock connector to USB cord.
Open iTunes and click on the name of your iPhone in the "Devices" section of the sidebar.
Click on the "Apps" tab along the top of the screen. Locate the app you want to put back on your phone in the "Sync Apps" list on the left side of the screen.
Check the box immediately to the left of the name of the app.
Click the "Sync" icon in the bottom right corner of the screen.