You can change the skill you're going to train as many times as possible during the week. However, what matters is the skill that you've set to train before the friday's training update takes place.
So for example, on monday you've set to train JS.
Tuesday you change your mind and change to handling.
Wednesday you change your mind and change to OD.
Thursday you change your mind and change to JR.
On Friday the training you've set is still JR, and the training update takes place. The skill that was trained would be the JR. So during the week, you can change as many times as you wont, but what matters is the skill that you've set to train before the Friday's training update.
I hope this is clear.