If Steam is not installed on your PC or console, download it from the official website -
here.
If Steam is already installed on your PC or console, you need to switch to a different account. In your current account, click
Switch Account rather than
Log Out. You will need to do this each time you want to sign in to a different account.
Next, enter the account credentials you received, make sure to check the
Remember me box, and click
Sign In.