Until August 2021, the latest iOS version for iPhone 8 is iOS14.7.1, which was released on July 26, 2021.
Compatible iPhone models are: iPhone 12/12 Mini, iPhone 12 Pro/Pro Max, iPhone SE (2nd), iPhone 11 Pro/Pro Max, iPhone 11, iPhone Xs/Xs Max, iPhone X/XR, iPhone 8/8 Plus, iPhone 7/7 Plus, iPhone 6s/6s Plus, iPhone SE (1st)
And the latest iOS 15 will be released in fall 2021, accompany with the latest iPhone 13. But you can download a beta version of iOS 15, which was released on June 7, 2021.
Note: Before upgrading to any iOS version, don't forget to back up your iPhone first, in case data loss.
