NOTE: You can sideload an app from any computer and not lose existing data (e.g. does not need to be performed on the computer your iOS device is synced with)
1. Open iTunes
2. Connect iOS device to computer and view inside iTunes by clicking on the name of the iOS device in the upper right-hand corner.
3. Click on the Apps tab.
4. In Windows Explorer, open the folder containing the .ipa file and double-click the file.
5. In iTunes, the app name will appear in the list.
6. Click the “Install” button. It should now say “Will Install” and the app icon should appear on the simulated home screen. (Alternatively, you can drag and drop the app icon onto the simulated home screen.)
7. Click “Apply” in the lower left-hand corner of iTunes.
8. Wait for the iOS device to finish syncing and then eject the iOS device from the computer.