Please watch the below video if you'd like to see how to do this (chapter 2).
Navigate to the Menu and click on Organisation and Users.
Click on the Users tab.
Locate the user that you need to amend the manager for by typing their name into the Search Users box and clicking on the magnifying glass.
Click on their username.
In the Common Tasks area, click on Set Line Manager or click on the Line Manager tab on the left-hand side.
Click on the Change button.
In the popup window, search for the Line Manager using the Filter and Search then select them.
Click on OK.
The line manager will be changed.