I believe you can't change the <kbd>Escape</kbd> keybinding, which is also used by many other operators.

But there are other (quicker) methods to exit circle selection.

From the [manual][1]:

> To cancel the selection use <kbd>![RMB][RMB] RMB</kbd> or key <kbd>Esc</kbd> or <kbd>&#9166; Enter</kbd>.

- **Press the <kbd>![RMB][RMB] RMB</kbd>.**
- If you're right-handed, awkwardly swivel your right thumb onto <kbd>Numpad Enter</kbd>.
- If you have a programmable mouse, assign any of its buttons to <kbd>&#9166; Enter</kbd> or <kbd>Escape</kbd>, this would be a useful keybind anyway which is not wasted.

  [1]: https://docs.blender.org/manual/en/dev/editors/3dview/object/selecting/tools.html#circle-select

  [RMB]: https://i.sstatic.net/LPwD4.png (Right Mouse Button)