I think I had forgotten to input a schooling deduction - I was wondering if I could repair my last years taxes, while doing this years?

If you need to make changes to your return from last year, you can do so. This TurboTax FAQ tells you how: I've filed my tax return. Can I still make changes?

If you had student loan repayments that you didn’t claim in previous years, you can claim them on this year’s return. The Canada Revenue Agency (CRA) allows you to accumulate 5 years’ worth of interest payments and claim them in one year. However, the student loan expense amount is non-refundable, meaning you can use it to reduce your taxes owing, but not create or add to a refund. So, you may want to claim the interest in a future year, if you don’t have taxes payable this year.

As per this TurboTax FAQ - How do I report student loans or a student line of credit?, this is how you can claim your Student Loan Interest in TurboTax Online:

1. Select Find (or the magnifying glass icon) from the top right
2. In the Find window, type Students and choose it from the list
3. On the Your Student Profile step, select Student loan interest and then Continue
4. On the Student Loan Interest step, enter your interest amounts

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