Changing Your Tax Return Once You Have Already Filed It.
If you have filed your return and then determine that you need to make a
change, either because you have received another T-slip, or because you didn't
claim an expense and later learned it was deductible, you can request an
adjustment to your tax return.
You can request the change online for your most recent return, or your returns for the previous two tax
years, or by mail, for tax returns for the past ten years.
Requesting A Change Online
Requesting a change online is very simple, and is done by logging into your
account at the
CRA My Account page. A separate request has to be filed for each
tax year.
You can also use the online request if you forgot to apply for the GST/HST tax
credit when you filed your tax return.
Requesting A Change By Mail
You can obtain a form T1Adj from the CRA web site, complete it and mail it in, along with documents supporting your change request. The time limit for filing adjustments to your tax returns by mail is ten years. An adjustment request may be made in 2008 for the 1998 or subsequent taxation years.