Either via the Steam menu at the top, or via your username, click and then choose "change user." That will completely log you out of your account, allowing you to log in to another account.

You misunderstanded, I loged in an account in Idle master and ticked "remember this computer". Then, I wanna log in another one, I could not change user in Idle Master. The problem is logging in an account in Idle Master within ticking "remember this computer" and no where can change user cuz the window log-in dissapear .

Idle Master shares it's cookie data with Internet Explorer. If you clear your cookie data from Internet Explorer, you should be able to log in a different user in Idle Master.