Please watch the below video if you'd like to see how to do this (Chapter 2).
Navigate to the Organisation and Users via the Menu.
Click on the Users tab.
Locate the User that you need to amend the Line Manager for by searching for them in the search box or locating them in the list of Users.
Click on their Username.
In the Common Tasks area of the User's Profile, click on Set Line Manager or click on the Line Manager tab on the left-hand side.
Under Line Manager, click Change.
In the popup window, search for the new Line Manager in the Filter field. Click the radio button next to their name to select them.
The Line Manager will be updated in the User's profile. Once set, you can remove a User's Line Manager by clicking Remove.