How to replace office chair wheel?
I have replaced those, but I had the fortune of working for a company that had spare seats with other broken casters, so when I pulled one off of one of those it did not matter.

The part that removes is not the part you have featured in your pictures. Those are the non removable axles. The point of separation is higher where the whole wheel assembly pivots. A strong direct pull down should separate it from the support arm that it pivots in.