1. Download and install Google Chrome web browser.
2. Logon with any of the account on the website that having multiple accounts, say Facebook.
3. Click on top right corner of browser and select "Settings".
4. Scroll down and click on "Add user".
6. Done, now you are using the chrome as another user. Open the same website but now login with another user id.
You can repeat these steps any number of times as far as how many accounts on perticular website you have!