Apple has announced iOS 8 at company's Worldwide Developers Conference (WWDC) 2014 on June 2, 2014. iOS 8 was officially 

released on September 17, 2014. According to reports iOS 8 has been installed on 83% of compatible Apple devices. There has 

been several version of iOS 8 with each version fixing bugs. There has been WiFi issue in initial launch of iOS 8 version. 

This resulted in sudden drop in WiFi connectivity leaving users in lurch. Apple launched subsequent version in short span 

fixing issue. As of today iOS 8.3 is available released on April 8, 2015. You can get iOS 8.4 beta version as well from Apple 

official site If you fall under 27% category of iOS users (iOS 7 and below) then surely upgrade to iOS 8. And to your 

surprise it is free !! Before upgrading to iOS 8 make sure your device is compatible.

Below are list of compatible device for iOS 8:

iPhone:

iPhone 4s and iPhone 5

iPhone 5c, iPhone 5s, iPhone 6 and iPhone 6 plus

2. iPad:

 iPad 2, iPad (3rd generation), iPad (4th generation), iPad Mini (1st generation)

  iPad Air, iPad Air 2, iPad Mini 2 and iPad Mini 3

3. iPod Touch:

 iPod Touch (5th generation)

You can install iOS 8 in all of above compatible device listed above. However, iPhone 4s, iPhone 5, iPad 2, iPad (3rd 

generation), iPad (4th generation) and iPad Mini (1st generation) are 32-bit device and soon will become obsolete.

Apple has recently forced developers to publish app on App Store build on 64-bit rather than 32-bit. This link Apple 

Developer will give you more details about 64-bit and iOS 8 requirement for App updates.