A topmost manager is the person in your organisation that all employees report to.
You must enable a user hierarchy for your account to create a topmost manager (see Creating a team structure (user hierarchy) in Sell).
You must have admin rights to create a the topmost manager of the account and the reporting structure of the users below.
After you have set up your user hierarchy, a user can only be changed to the topmost manager in the account if they do not have other users reporting to them.
Click on the user's name to edit their permissions, and you'll see the make topmost manager option.
A user with no reportees can be made the topmost manager. If a user has reportees, there is no option to make them the topmost manager.
For more information about creating groups and teams in Sell, see Working with groups and teams in Sell.