Wt is upper calss english

Upper class English is formal / proper English spoken / written in a posh way.
It shows others that you are well educated and polite (I am not so sure of this politeness, for it is possible to cause vexations even with upper class English).

You may begin to read some of the classic novels such as Jane Austin's Pride And Prejudice, Sense And Sensibility, Persuasion, Mansfield Park and Emma; Sir Arthur Conan Doyle's Sherlock Holmes series; and Charles Dickens' Oliver Twist, A Christmas Carol and Our Mutual Friends.

In case you have not noticed, I have attempted to write the above in posh English, and hopefully it would provide you with some concepts of upper class English.
God Bless!

Agreed with 小安安. But upper class English is also requested to have certain accents in speaking, usually called "the Queen language".

Queen English" is Received Pronunciation (RP) is a form of pronunciation of the English language, commonly spoken Her Majest y ,Queen Elizabeth II & her Royal family. RP sometimes defined as the "educated spoken English of southeastern England." It is a dialect of English often taught to non-native speakers,Many Britons abroad modify their accent to make their pronunciation closer to Received Pronunciation, in order to be better understood than if they were using their usual accent. They may also modify their vocabulary and grammar to be closer to Standard English