You misunderstood.
My OS (Windows 10 Pro 64-bit) is completely fine and already 64-bit so no need to update and/or upgrade it.
The problem is the QuickBooks software.
This is the one running 32-bit version while it should (probably) run 64-bit one (if exists).
That's why I was asking where I can find a 64-bit version of the QuickBooks Desktop Pro 2019.
Can you please just send me a direct link to download it?
You said "On the other hand, 64-bit refers to the Windows operating system. "  - This is completely incorrect as every app and/or program can also come in two versions in respect to file system (32 or 64-).
I am an IT expert so don't need to reach out to one...
As for the first part of your answer: I had done this workflow (locating and killing/ending the running process of the program) many times before I posted in the forum, however, it had not helped to resolve the problem.
I had also updated the file to the latest release before I posted here. 
So in all, none of what you suggested worked. 
By the way in the Task Manager, when you click on the process you don't click on a file (as you wrote) but on a process, the program is running, not its file.
Update: By the time I am writing this reply I already resolved the problem. I changed the location of the Company File and renamed. Then the QB successfully opened it.