You can amend your original online application by logging back in at: sheffield.ac.uk/postgraduate/online. You can change your contact details and course choices (by adding or removing choices), and you can request a deferral if you'd like a later start date.
A change of start date is at the discretion of the University. It isn't automatically given and you may be asked to make a new application. Remember that most postgraduate taught courses have one admission per year, at the start of the autumn semester in September.
Please note that changing your email address in your online application will then change the email address you should use to log in.