while updating it says,update ios to 8 but it cannot. and now i haveing trouble downloding facebook and all.... how can i update or download facebook and all????

What model iPhone do you have? You need an iPhone 4s or newer to run anything beyond iOS 7. If you have an iPhone 4 or older, you cannout update to iOS 8 or any iOS newer than 8.

That is an iPhone 4. It cannot be updated beyond iOS 7.1.2. If you need apps that require a newer version of iOS, then you will need to purchase a newer iPhone.

I upgraded my I phone to to iPhone to IPhone 8 i have a problem with facebook

And that problem would be what?