From your 'My Hub' page (please see below)
On the left hand side of your account, you will see the side menu where you can select 'Workouts'.
This will show you any workouts you have created.
Clicking the 3 horizontal grey dots next to any workout you will then have the option to view the workout, as well as 'Assign and Unassign' it to any of your clients.
If you click on 'assign clients' you can then select the tickbox to assign the workout to that client or unselect it to un-assign it and then hit 'Save'.