You can't. (To the best of my knowledge.)
It is one of the ways QBs makes you update.
NOTE: You may not want to install QB2020 over your existing QB2019!! You may want to install it into a separate file and keep your QB2019 installation working as well. (Advanced option and create a new folder.)
Since you said "client" I'm assuming you have other clients. If you open one of your QB2019 client's files in QB2020 it will update that file and they will not be able to open it in their system! It's fine if you're just opening it on your machine. But if you're making entries/corrections for them and expect to give the file back, be careful not to update it to the latest version.
