Can't update angular to version 6

Just to be sure run the following steps:

npm uninstall -g @angular/cli
npm cache verify
npm install -g @angular/cli@latest

Then in your Local project package:

rm -rf node_modules dist 
npm install --save-dev @angular/cli@latest
npm i 
ng update @angular/cli 
ng update @angular/core

If this doesn't work, I think you should update manually the package.json

  "dependencies": {
    "@angular/animations": "^6.0.3",
    "@angular/common": "^6.0.3",
    "@angular/compiler": "^6.0.3",
    "@angular/core": "^6.0.3",
    "@angular/forms": "^6.0.3",
    "@angular/http": "^6.0.3",
    "@angular/platform-browser": "^6.0.3",
    "@angular/platform-browser-dynamic": "^6.0.3",
    "@angular/router": "^6.0.3",
    "core-js": "^2.5.4",
    "rxjs": "^6.0.0",
    "zone.js": "^0.8.26"
  },
  "devDependencies": {
    "@angular/compiler-cli": "^6.0.3",
    "@angular-devkit/build-angular": "~0.6.5",
    "typescript": "~2.7.2",
    "@angular/cli": "~6.0.5",
    "@angular/language-service": "^6.0.3",
    "@types/jasmine": "~2.8.6",
    "@types/jasminewd2": "~2.0.3",
    "@types/node": "~8.9.4",
    "codelyzer": "~4.2.1",
    "jasmine-core": "~2.99.1",
    "jasmine-spec-reporter": "~4.2.1",
    "karma": "~1.7.1",
    "karma-chrome-launcher": "~2.2.0",
    "karma-coverage-istanbul-reporter": "~1.4.2",
    "karma-jasmine": "~1.1.1",
    "karma-jasmine-html-reporter": "^0.2.2",
    "protractor": "~5.3.0",
    "ts-node": "~5.0.1",
    "tslint": "~5.9.1"
  }

To remove .angular-cli.json and create angular.json if updated manually

  ng update @angular/cli --migrate-only --from=1.7.4

Try this answer: https://stackoverflow.com/a/50965027/1002641

The problem was having ng installed under /usr/local (and a symlink in /usr/local/bin). Removing it, helped me update my @angular/cli version to 6 (from 1.7.1)