If you have been asked to confirm a shift that you don't want to or can't work, you can follow the steps in the below article to decline it. You will have received an email with a link where you can go directly to the shift details to decline your shift.
What if I've confirmed already and I now want to cancel?
Once you've confirmed your shift, you can't then usually cancel it yourself. If there is no cancel button, you'll need to get in contact with your agency and let them know that you need to change the confirmation status. You can see how to contact your agency by clicking the below link:
What if I've confirmed but I want to change the shift times?
As above, you can't usually change the shift times beforehand without speaking to your agency first and them changing it for you. Please contact your agency and they will be able to discuss with you whether it's possible to change the times.