*nods*
You can apply from any time within registrations open. You get all the details/information you need.
You can change your preferences before/after your UAI although they do have restrictions, something like preference changes have to be done before a certain date time, before a round commences.
Your best off doing your preferences the courses you want in the EXACT order you want no matter if you think you will get that UAI or not. Because well you never know how many people will apply, how the UAI for the course might lower or higher, what your UAI will be etc etc.
A lot of people get caught out that way so be warned put in order what you want to do no matter what. Because if you don't get your 1st preferences they will go to your second, third and so on.
Happy deciding.
Otherwise go read up on the UAC website I am sure they have all the rules/procedures up there somewhere.