Edit a custom exercise

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 'Exercises'.

 Select the 'Custom Exercises' tab from the 2 options at the top of your page.

Here you can click the 3 horizontal grey dots to the right of any custom exercise to 'Edit'.

This is where you can then edit the custom exercise.