In QBO, each company file is a different account. You have to be added as a user to the other company files. After that, you should be able to switch between accounts.
To do this task, you also need to be the Company Administrator of the other file. And follow the steps below:
1) Choose Gear icon at the top, then select Manage Users under “Your Account” section.
2) Click the Add user button on the right.
3) Select which type of user you’d need to be added:
4) Regular or custom user and click Next:
5) Select what access rights you’d like them to have, click Next
6) Set the users admin rights, click Next.
7) Enter your user e-mail address and name, click Save.
8) Company administrator and click Next:
9) Enter your user e-mail address and name.
10) Click Save.