Thank you for your question.

You can change your SIN in the Intro section of your return if you have not already certified or submitted your return to the CRA.

If you have already certified your return, you will need to make a copy of the return by clicking Tools in the upper right hand corner and selecting Return Manager from the drop down menu. Here, you can create a copy of your return to change your SIN in the Intro section.

If you have mailed a return with an incorrect SIN to the CRA, kindly contact them as soon as possible: http://www.cra-arc.gc.ca/cntct/menu-eng.html