Downloading apps to your iPhone is usually a quick and simple process. However, you may sometimes have problems getting them to install. Before you do anything, make sure that you're downloading an Apple-approved app from the App Store and that your phone has a steady cellular or Wi-Fi connection. If you tick both those boxes but still can't install, troubleshoot the download by trying it again, running checks on your iPhone or using iTunes as an alternative.
Delete and Download Again
You may be able to solve a download problem by deleting the app and starting over. Press and hold down the app's icon until it "jiggles." Tap the "X" and then "Delete" on the confirmation screen. Press the "Home" button to turn off the jiggle, tap "App Store" and go back to the app's page. Tap the download button to restart the process -- you don't have to pay for it again if you're using the same Apple ID.
Try a Different App
Sometimes, the app itself causes installation problems -- see if this is a problem by trying to download a different one. Go to the App Store, tap "Top Charts" on the screen’s bottom menu and then select "Free." Choose an app and start the download process. If it works, the original app is the problem -- if you really want to find a fix, you can usually find developer contact details in the information area of the app's description. If you can't download this app either, you have an iPhone issue.
Check Your Storage Capacity
You can't download an app unless you have enough space on your iPhone. To check capacity, note the app's file size from its information screen in the App Store. Go to "Settings,” tap "General" and then "About." In the Available field, check how much free space you have. If this is smaller than the app file, get extra space by removing unwanted apps or content, importing photos and videos to your computer or backing up and syncing to iTunes or iCloud. Keep in mind, Apple only stores certain types of content, so be careful not to delete anything that you can't get back.
Try Syncing with ITunes
Sometimes, syncing your iPhone on your computer kickstarts and completes a download. Open iTunes, connect your iPhone and select it in the Devices list. Click "Sync" on the Summary page. When syncing is complete, select "Apps" and see if the app is in the list. If it is, and you don't have automatic installation enabled, drag it on to one of your screens or select "Install." Select "Apply" to put the app on your iPhone.
Restart and Reboot Your IPhone
Sometimes, turning your iPhone off and on again fixes bugs that might affect downloads. Hold down the "Sleep/Wake" button and swipe the slider to the right. When the phone shuts down, hold down the "Sleep/Wake" button to restart it. If this doesn't work, try rebooting the phone -- although this shouldn't cause data loss, back up your iPhone first. Hold down both the "Sleep/Wake" and "Home" buttons until the Apple logo appears and wait for your iPhone to reboot and restart.