Only one return per tax year will be accepted by the Canada Revenue Agency. You can't file a new return once you've filed your previous one (mistakes and all). However, you can make a change to the original to correct it.
There are two ways to adjust your return
Online changes: After signing into your account, click the “Change return” option from the left side. You’ll choose the tax year you want to adjust and enter line numbers and figures to correct or add information
Changes by mail: If you prefer to submit your adjustment request by paper, fill out a Form T1ADJ – T1 Adjustment Request and mail it to your local tax centre