Tax return with 10 years can be adjusted after it is filed to CRA and an assessment is received. If you just filed the return, please wait for the assessment to come, then you can adjust it.
There are several ways to amend a tax return:
- Fill form T1-Adj and mail it to CRA; or
- If you use Netfile to file return, you can login into CRA portal "My Account" to change tax return.
- If you are an EFiler, you can also register as level 2 representative using T1013, then login to CRA protal "Represent a client" to change tax return.
Please read this CRA page to get instruction details.