Isolate the circuit and cut the wire that you want to remove about an inch or so from the fitting. Remove the leftover insulation and use a small pair of pliers to grab the bare wire, twisting it in one direction while pulling on it.
Those push connectors aren't meant to be reused. You could, but there's a very good chance they won't grab like before, and loose connections are bad news.
I'd cut the wire as close as possible, and use those empty spots you got if you're reinstalling that fixture.
Those push lock connectors are the cat's meow for solid wire. Absolutely love them. But yes, generally you can grab them and twist and pull and they will come back out. They are supposed to be one-time use though. They're called wall-nuts, or some guys called them Wago's as that's the main brand of them.
They might be OK to reuse when new, but for the few pennies they are, absolutely not worth reusing after a while.
I use them at work a lot, and I'd absolutely not take the chance reusing them even with solid wire. Like I said, even the brand name stuff are pennies, and in this case he's got empty spots, don't bother reusing them.