Look on the financial aid webpage for each college and determine the submission deadline for the school. But, be aware that some scholarship programs may have earlier deadlines or award funds on a first-come, first-served basis.
To repeat: you can't. For the 2020-2021 FAFSA, you will have to use 2018 income. You will be able to use your anticipated lower 2019 income when you complete the 2021-2022 FAFSA.