You will only be able to reassign clients and their reports to another user within your account if view all user's clients / reports has been activated for your account.
It is worth bearing mind that when you change the ownership of a client to another user, the reports attached to that client will be automatically switched to that user too.
To reassign multiple clients and their reports to another user within your account, go to Client & Report Library from the Libraries dashboard, select the clients you wish to reassign using the checkboxes in the first column, and click the 'Reassign X selected clients' button.
This will open a new window. Simply select the the user you wish to reassign the clients to and click Save.