I have also faced the same issue in recent past for me I have do the following commands one by one in terminal.

sudo npm uninstall -g angular-cli
sudo npm cache clean

After this run

ng -v

If still get angular-cli version 1.0.0-beta.2x.x then run the following command

which ng

It will show the ng path. Go to the path and if it is linked with any file remove the same the link and actual ng file. In my case the link is in /usr/bin/ng and actual path of ng file is /lib/node_modules/@angular/cli/bin/ng.

sudo rm -rf /lib/node_modules/@angular/cli/bin/ng
sudo rm -rf /usr/bin/ng

Next you need to install @angular/cli using

sudo npm install -g @angular/cli

Close all the terminal and run ng -v and you are on. May be it will help someone. Thanks :)

If you are facing issue with angular/cli then use the following commands:
npm uninstall -g angular-cli to uninstall the angular/cli.
npm cache clean to clean your npm cache from app data folder under your username.
use npm cache verify to verify your cache whether it is corrupted or not.
use npm cache verify --force to clean your entire cache from your system.

Note:
You can also delete by the following the paths
C:\Users\"Your_syste_User_name"\AppData\Roaming\npm and
C:\Users\"Your_syste_User_name"\AppData\Roaming\npm-cache
Then use the following command to install latest angular/cli version globally in your system.
npm install -g @angular/cli@latest
To get more information visit github angular-cli update.

Updating Angular CLI

https://github.com/angular/angular-cli#updating-angular-cli

If you're using Angular CLI 1.0.0-beta.28 or less, you need to uninstall angular-cli package first.

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

Then when it gets done successfully you may try:

ng --version