Before you can change the ownership of a client to another user within your account view all user's clients / reports will need to ahve been activated for you. When you change the ownership of a client to another user within your account, the reports attached to that client will be automatically switched to that user too. 

To change the ownership of a client, go to Client & Report Library from the Libraries dashboard, and locate the client using the search function at the top of the page. 

Next, click the edit pencil to the right of the client's name and a new window will open. Simply select the user you wish to change the ownership to in the 'Owned by' field, click Save, and then close the window.