1.卸载之前的版本

npm uninstall -g @angular/cli

2.检查是否卸载干净,输入命令

 ng version

若显示 command not found: ng 则卸载干净

3.安装指定版本

npm install -g @angular/cli@8.0.6

5.检查版本号

ng version

     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/


Angular CLI: 9.1.12
Node: 14.5.0
OS: darwin x64

Angular: 9.1.12
... animations, cli, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router
Ivy Workspace: Yes

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.901.12
@angular-devkit/build-angular     0.901.12
@angular-devkit/build-optimizer   0.901.12
@angular-devkit/build-webpack     0.901.12
@angular-devkit/core              9.1.12
@angular-devkit/schematics        9.1.12
@ngtools/webpack                  9.1.12
@schematics/angular               9.1.12
@schematics/update                0.901.12
rxjs                              6.5.5
typescript                        3.8.3
webpack                           4.42.0

显示这些信息，恭喜你，表示你已经安装成功；