Capital loss carried back to prior year

I have a capital loss that occurred in the current year and which I want to carry back to a prior year. Do I have to change the client's data in the file from that year?

No. Use the keyword Carry-Back in the current year's file, and indicate the amount of the loss that you want to carry back. DT Max will then generate the T1A form (TP-1012 in Quebec) which you must attach to the return.

Do not modify a return from a prior year. The government will make the necessary adjustments.