Please watch the video below if you'd like to see how to do this.
Navigate to the Organisation and Users via the Menu.
Click on the Users tab.
Locate the User that you want to set 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.
Click on Set Line Manager.
In the popup window, search for the User you wish to set as Line Manager in the Filter field. Click the radio button next to their name to select them.
The Line Manager will now be set in the User's profile. Once set, you can remove a User's Line Manager by clicking Remove.