If it is an Apple phone, it already is installed. If not, then you can't install it. If it is an iPhone and the flash memory has been completely wiped, then you would at least have to go to an Apple Store to get it re-flashed. If you are in Bangldesh or in an area where Apple Stores don't exist, you would have to send it to someone in an area where one does exist.
Before starting, bear in mind that you'll need to be operating iOS 7 or more recent to be able to upgrade over-the-air. And you'll also need to create sure your system is reinforced. These are the iPhone and iPad designs able of operating iOS 8:
iPhone 4s and newer
iPad 2 and newer
iPod contact 5th generation
Back up your iPhone or iPad with either iCloud or via iTunes.
Make sure you're on the newest edition of iTunes. Go to About and then Examine for Up-dates or check out iTunes.com.
Launch iTunes on your Mac or PC.
Plug in your iPhone, iPad, or iPod contact if it isn't already.
Click on iPhone, iPad, or iPod in the top right routing.
Click on the Examine for Update key under the Conclusion tab — at the same time, if you'd like to reinstate your system fresh and fresh everything you'd simply select the Recover choice instead.
An upgrade should be identified, simply just click Obtain and Set up in the pop-up selection and accept to any conditions or conditions