Directly From your Phone, IPod or IPAD
Step 1 - On your iPhone/iPod Touch/iPad open an app called Settings
Step 2 - Click General from the list
Step 3 - Then click on Software Update
Step 4 - Click Download new iOS 8
Step 5 - Wait for it to download an then click install
or
From Your Computer
Step 1 - Connect your iPhone/iPod Touch/iPad to your Mac computer
Step 2 - A message will appear for New firmware iOS 8 update
Step 3 - Click Download and Install
Step 4 - leave it connected while it installs iOS 8.