Tax return 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 two ways to file T1 adjustment (T1-Adj):
1) file paper form T1-ADJ to CRA or
2) login to CRA online "my account" service to change it through Internet.
Either way, you need to read this CRA page to get instruction.