You are free to cancel any appointment you have made provided you have given at least 48 hours' notice.

This could potentially result in forfeiting your weekly slot and having to find a different time for the weeks that follow if you are looking to continue with the same therapist, but it is up to the individual therapist to decide if this will be the case. It's likely that the therapist will simply just book your next session for the next date you are available as long as this is discussed in advance, with more than 48 hours notice, and is not a frequent occurrence. 

Please be aware that if you cancel or request to rearrange sessions often or regularly, your therapist may need to decline any future bookings you make.