I answered no to the previous refusals question by mistake. Can I fix it?

Yes, and the timing is the part that matters. A correction made before a decision and a correction made after one are not the same thing.
Start with what the Act actually says, because the fear around this is usually bigger and vaguer than the provision. Section 40(1)(a) makes a person inadmissible for directly or indirectly misrepresenting or withholding material facts relating to a relevant matter that induces or could induce an error in administering the Act. Note what it turns on. Withholding. A fact that was asked for and not given.
Where a finding is made, section 40(2)(a) sets the consequence at five years, and section 40(3) prevents a foreign national who is inadmissible under that section from applying for permanent resident status during it.
The practical thing people miss is that IRCC already holds your refusal history. It is on their system. When an officer opens your application, a note next to two refusals on file is visible immediately, and there is no version of events where they do not see it. The question was never whether it would come out.
The correction goes through the IRCC web form. Choose the option for adding documents or information to support or complete an application, enter your application number and your details, and upload a short signed letter. State plainly that you answered no in error, that the correct answer is yes, and give the month and year of each refusal, the office that issued it, and the reason you were given. One page. Do not argue the old refusals or explain them away, because that is not what the letter is for.
Two things not to do. Do not withdraw the application and start again hoping for a clean file, because the refusal history follows you regardless and a withdrawal immediately after submission is itself visible. And do not wait to see whether anyone notices.
After you send it, expect nothing beyond the automatic receipt. Silence is normal. The letter sits on the file and the officer sees it when they open your application.
One last point that gets forgotten after a correction works. It fixes the answer, not the history. Those refusals still get declared on every application you make afterwards.
Sources: Immigration and Refugee Protection Act, section 40, current to 30 September 2026. IRCC, Your IRCC application: update your application, canada.ca, updated 3 September 2026.
This is general information rather than legal advice, and it reflects those sources as they stood on the dates above. Where a misrepresentation issue is already live on your file, that is a question for a licensed professional rather than a forum.