1.首先根据网上的教程，先卸载低版本的cli，再升级cli3.如下：
C:\Users\feng>vue -V
2.9.6
C:\Users\feng>npm uninstall -g @vue/cli
up to date in 0.039s
C:\Users\feng>npm install -g @vue/cli
npm WARN deprecated request@2.88.2: request has been deprecated, see https://github.com/request/request/issues/3142
npm WARN deprecated urix@0.1.0: Please see https://github.com/lydell/urix#deprecated
npm WARN deprecated resolve-url@0.2.1: https://github.com/lydell/resolve-url#deprecated
npm WARN deprecated chokidar@2.1.8: Chokidar 2 will break on node v14+. Upgrade to chokidar 3 with 15x less dependencies.
npm WARN deprecated fsevents@1.2.13: fsevents 1 will break on node v14+ and could be using insecure binaries. Upgrade to fsevents 2.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@^1.2.7 (node_modules\@vue\cli\node_modules\chokidar\node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.13: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})
npm ERR! code EEXIST
npm ERR! path C:\Users\feng\AppData\Roaming\npm\node_modules\@vue\cli\bin\vue.js
npm ERR! dest C:\Users\feng\AppData\Roaming\npm\vue
npm ERR! EEXIST: file already exists, cmd shim 'C:\Users\feng\AppData\Roaming\npm\node_modules\@vue\cli\bin\vue.js' -> 'C:\Users\feng\AppData\Roaming\npm\vue'
npm ERR! File exists: C:\Users\feng\AppData\Roaming\npm\vue
npm ERR! Remove the existing file and try again, or run npm
npm ERR! with --force to overwrite files recklessly.
npm ERR! A complete log of this run can be found in:
npm ERR! C:\Users\feng\AppData\Roaming\npm-cache\_logs\2020-05-09T01_22_24_602Z-debug.log
C:\Users\feng>vue -V
2.9.6可以看到报错了，原因出在卸载的那一步，压根就没删除掉老版本
2.尝试使用管理员权限执行
C:\WINDOWS\system32>npm uninstall -g vue-cli
removed 241 packages in 3.235s
C:\WINDOWS\system32>npm install -g @vue/cli
npm WARN deprecated request@2.88.2: request has been deprecated, see https://github.com/request/request/issues/3142
npm WARN deprecated urix@0.1.0: Please see https://github.com/lydell/urix#deprecated
npm WARN deprecated resolve-url@0.2.1: https://github.com/lydell/resolve-url#deprecated
npm WARN deprecated chokidar@2.1.8: Chokidar 2 will break on node v14+. Upgrade to chokidar 3 with 15x less dependencies.
npm WARN deprecated fsevents@1.2.13: fsevents 1 will break on node v14+ and could be using insecure binaries. Upgrade to fsevents 2.
C:\Users\feng\AppData\Roaming\npm\vue -> C:\Users\feng\AppData\Roaming\npm\node_modules\@vue\cli\bin\vue.js
> core-js@3.6.5 postinstall C:\Users\feng\AppData\Roaming\npm\node_modules\@vue\cli\node_modules\core-js
> node -e "try{require('./postinstall')}catch(e){}"
Thank you for using core-js ( https://github.com/zloirock/core-js ) for polyfilling JavaScript standard library!
The project needs your help! Please consider supporting of core-js on Open Collective or Patreon:
> https://opencollective.com/core-js
> https://www.patreon.com/zloirock
Also, the author of core-js ( https://github.com/zloirock ) is looking for a good job -)
> @apollo/protobufjs@1.0.3 postinstall C:\Users\feng\AppData\Roaming\npm\node_modules\@vue\cli\node_modules\@apollo\protobufjs
> node scripts/postinstall
> nodemon@1.19.4 postinstall C:\Users\feng\AppData\Roaming\npm\node_modules\@vue\cli\node_modules\nodemon
> node bin/postinstall || exit 0
Love nodemon? You can now support the project via the open collective:
> https://opencollective.com/nodemon/donate
> ejs@2.7.4 postinstall C:\Users\feng\AppData\Roaming\npm\node_modules\@vue\cli\node_modules\ejs
> node ./postinstall.js
Thank you for installing EJS: built with the Jake JavaScript build tool (https://jakejs.com/)
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@^1.2.7 (node_modules\@vue\cli\node_modules\chokidar\node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.13: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})
+ @vue/cli@4.3.1
added 1169 packages from 668 contributors in 649.52s
C:\WINDOWS\system32>vue -V
@vue/cli 4.3.1
C:\WINDOWS\system32>已经升级完成了
由于我的nodejs版本比较高，直接升级到4.3.1了，也可以指定版本升级！