If you are using the same App Store account you should download the same version you previously had. If that was the paid app, download that again for free. If it was the free app, download it again, tap on the upgrade notice and then tap Restore Purchases.
Data is stored within the app. Deleting the app will delete the data along with it. The exception to this is whether you had enabled iCloud Drive and Seconds within your iCloud settings. If this was the case you will be able to restore your timers from the iCloud Backup section within the Settings.