I used TurboTax online to do my 2016 tax return, but I missed a couple T5s . CRA has asked me to complete or update a Schedule 4; I don't know how to do that.

If you successfully filed your return, you can't file again. When you need to make corrections to a filed return, you typically need to prepare what's called an adjustment. Here's how you do that: https://turbotax.community.intuit.ca/replies/2647992