Not sure why php APCu is not a part of uniform server, it was before.
(so you could only edit php.ini file and uncomment a line to enable it, or just select it in the UI under PHP Accelerators).
Perhaps because they ship PHP 8.0 now, and in the early days of 8.0 it wasn't supported. (I think it is now).
So I have no idea how to do it in Windows, you can google around, perhaps see UniServZ forum, they have a support board there. And probably some tutorials on how to manually install php extensions.
In Linux, its just one install command, and a restart command if it doesn't do that automatically and your good to go. x)