To assign a manager to an employee via their profile, navigate to their Job tab. At the very top of the page, you will see the first setting under employee name's job details is Manager. Select their manager from the drop-down list.

Once you have done this, you will see a notification appear at the top of your screen confirming that the change has been saved. 

Managers can also be assigned via the organisation chart. To do this, open the org chart and locate the employee you would like to assign a manager to. Once you have done this, click on the cog. This will bring up a new section where you can choose the new line manager for that employee from the manager dropdown menu. Click update details to save changes.