I have a client that uses QB Pro 2016 on her computer. I currently use QB Pro 2019 on mine. Is it possible for me to upload a backup copy of her company file, update it on my computer (QB 2019) and then save a backup copy for her to upload back onto her computer (QB 2016) and it work properly?
You cannot back save into an older version. Choices. Client must upgrade to same version as you. Or you have to use your own 2016 version if you ever had it (#1 reason to install newer versions alongside older and not replace them)