This doesn't happen usually but just in case apps get stuck at “Waiting” when you try to install or update them, there's really nothing much you can do but to cancel the download and restart it.
The problem is not uncommon though. There are certain cases where this might happen all too often. Network, available free space and apps running the background can all play a role in this issue.
Let's check out how to fix this problem:
Check Your Network
First off, if you're on 3G/LTE, you might want to make sure there aren't network issues. If you're on Wi-fi, check to see if the Wi-fi is stable and doing good. A majority of the problem of apps getting stuck on “Waiting” is mostly because there is no stable data network.
Once you've fixed this, you might still have to re-download the app. Delete the app that's currently installing (hold on it and when it jiggles, press the delete button). Re-install it from the App Store again.
Space on the iPhone
Head over to General → Usage → and make sure you've got enough free space. This isn't usually the problem but if you store quite a lot of music, videos and other app data on the iPhone, you might just run into problems when you download an app.
Also, if the app that's stuck on “Waiting” shows up on this list, you should tap on it and click on Delete App. This will clear the download and you can re-download the app. This time, it should be successful.
Close Background Apps
The other reason – not proved but suspected – is a lot of apps running in the background. If you've got a bunch of them running in the background, this slows down the system quite a bit. To solve this, go to the multitask switcher by double-pressing the home button and then removing all apps that are running in the background.
Once cleared, the iPhone usually is now free enough to process faster. Try downloading the app now.
What if this still doesn't solve your problem!?
There are a bunch of things to check:
- App Store is slow: may be it's a server issue. Try again after a bit.
- iPhone is low on resources. Kill all apps and respring/reboot your iPhone.
- Try installing it via iTunes instead of directly on the iPhone. Use the Sync option.