Logging in to your customer's account.
Once you've given yourself access to the managed company (How to give yourself and users access to a managed company), you can access their account by clicking on the drop-down in the top-right corner of your own account.
You'll then be presented with a list of your Child Company's. You can do anything that these companies can do.
Once you're finished in the end user account, just click on the drop-down in the top-right corner of the page again and select your own company name.
Managing your customer's account externally
Click on your name in the top right.
Click on Partners in the drop-down menu.
Then click on the name of the company you'd like to manage.
From here, you can view the Company information, and from the options in the top-right, you can view its Asset, Run Books (only relevant for SIEM customers), or even Edit the Company's information.