Because assigning a programme copies it, you have two places to make changes.
For this client only
Open the client's calendar and move, swap, edit or remove the entries directly. This is the right choice for adjusting around an injury, a holiday or a change of race date.
For everyone you assign it to in future
Open the programme under Library → Programmes and edit the day grid. Clients who already have the programme are not changed.
Tip: If a client needs the updated version, remove the old entries from their calendar and assign the programme again with the same start date.