Go to the 90 day reporting website.
There is a kind of use's guide available from the link at the bottom of the login window. (a bit of old, but main ops are covered)
You need to register your account first.
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usually you can do 90-days report online when you move inside the same province. You can check with your new local immigration office for their policy. You have online reporting window from 15 days before and until the due date. You also have 7 days after for reporting in person. So you can try online first to see whether your report is approved. If rejected. go in person, you still have time.