If your iPhone acts up, here's how to take action.
Force Quit Frozen Apps
The iPhone is a very impressive piece of technology, but it can run into speedbumps from time to time. If you’re in an application that has become unresponsive or freezes, try to Force Quit, just like you would on a computer. Hold down the Home button in the iPhone face for six seconds, and it should quit the program and take you back to the Home screen.
Reset a Crashed iPhone
If a frozen iPhone won’t respond to a Force Quit, don’t panic; reset it to put it back in shape. Hold down the Home button on the face and the Sleep/Wake button on the top-right of the iPhone for ten seconds, and release them. The screen will go black, and then the Apple logo will appear as the phone boots back up again. If the phone turns off without restarting, hold only the Sleep/Wake button to force it on again.
Install New Copies of Troublesome Apps
With all of the new Applications in the App Store on iTunes appearing every day, buggy software can sometimes slip through. If you find that one particular piece of software continues to cause problems, try downloading it again through iTunes. This does a clean install on your iPhone and will often fix problems. The good news is, you won’t have to pay a second time for anything you’ve already downloaded. Just download as before, and when the screen comes up asking if you want to download the item again for free, hit OK. Sync the iPhone after completing the download.
Restore When All Else Fails 
If the iPhone has a catastrophic failure and won’t respond to other troubleshooting attempts, restore it through iTunes. Connect your iPhone to your computer, and when your iPhone appears in the sidebar, right-click (PCs) or Control-click (Macs) on the device, and select “Restore from backup.”
iTunes normally backs up the phone when you sync, so you should have a recent version of everything. (You can manually back up using the same Control- or right-click menu.) Additionally, iTunes might give you the option to back it up now before continuing. Click Back Up. Depending on the amount of data on the phone, the process might take about a half-hour. Afterwards, iTunes will restore the phone with Apple’s latest firmware. Including the automatic firmware download, if needed, the restore can take an hour or more.
When complete, iTunes will give you the option to restore your data from a backup. Pick the most recent option, or if you’ve already tried a restore and the phone still had problems, reach back to an earlier state. It can take another half-hour or more to transfer all of your data back to the phone, but when complete, your old friend will be back to normal.
Enter DFU Mode for a Complete Restore 
If other recovery techniques still don’t work to fix a glitchy phone—especially if you can’t even get far enough to activate a regular restore—use Device Firmware Upgrade mode to completely start over. Attach the phone to your computer, and hold down the Home button and the Sleep/Wake key like you do in a reset. However, when the iPhone’s screen goes black, hold them an extra two seconds, and then release the Sleep/Wake key, but keep holding down the Home button for ten seconds. The phone will appear in iTunes as a blank slate without applications, music, contacts, or any data. Restore the phone as you would otherwise.
Check out our other iPhone University subjects:
Fine Photos - Shoot—or at least end up with—snappier pictures with the built in camera
Email Expertise - Manage, send, and receive messages easier than ever
Instant Interface - Learn input secrets for any situation.
iTunes - Make your music library and iPhone sing together.
