I submitted my 2019 tax returns last month and had a substantial capital gains, so I had to pay taxes on it.
In 2020, I have had a capital loss on some of my investments. Am I able to use the 2020 capital losses by changing my 2019 returns now and resubmitting them or will I have to claim the losses until I do my 2020 tax returns?
Since your capital losses were in 2020 and not in 2019, they should be filed in your 2020 tax return.
I believe you will have to file Form T1A to request for Loss Carryback when you file your 2020 return.
More information can be found on the CRA website:
The site above also mentions to not file an amended income tax and benefit return for the year to which you want to apply the loss. Therefore, you shouldn't amend 2019 with 2020 losses.