If you’ve successfully filed your return to CRA’s NETFILE service and received a confirmation number, you cannot make a correction and file again using CRA’s NETFILE service. Instead, you’ll need to file an amendment. Follow these steps: 

1. Check that the CRA received your return

2. Follow these instructions to amend your return 

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/chngrtrn-eng.html