I know BlockOne added a tool online, I know some developers did a nice Web page but you shouldn't trust the browser or an external developers to generate a private key!!!

You can still update your EOS address in the smart contract before the mainnet launch of june.

1. Install NodeJS

it's available for Linux, Mac and Windows. https://nodejs.org/en/

2. Install eosjs-ecc to generate the EOS keys with JavaScript

Open a terminal and type:

npm install eosjs-ecc

It comes from the official EOSIO github project https://github.com/EOSIO/eosjs-ecc

Now you can turn OFF the Wifi or the internet connection and type:

node

3. Generate an EOS private key

Cut and paste this, to generate a new keypair

let { PrivateKey } = require('eosjs-ecc')

PrivateKey.randomKey().then(privateKey => {
  let privateWif = privateKey.toWif()
  let pubkey = PrivateKey.fromWif(privateWif).toPublic().toString()

  console.log(`
    EOS private: ${privateWif}
    EOS public: ${pubkey}
  `)
})

4. Verify an EOS private key

Add your private key and cut and paste this to verify it

let { PrivateKey } = require('eosjs-ecc')

function verify(privkey) {
  return PrivateKey.fromWif(privkey).toPublic().toString()
}

// PUT BELOW YOUR PRIVATE KEY
var privkey = '5K4zLkAZGWTK7xZpYoy2NLT6nUNSnhDZqLSDRM7WhuXatKzaPUA'
console.log(`
  EOS public: ${verify(privkey)}
`)

Congratulation! If the output is the public key you have everything is fine!