Windows doesn't officially support changing the default file manager so it is not available in Settings. If you are running a version of OneCommander downloaded from the website (instead of Store version), then you can register it as default from Settings>Advanced. Please just read the warning that shows when you click it, and know that if you had some other file manager registered as default in the past (other than Microsoft File Explorer) there might be some registry conflicts and unpredictable results. Also, this replaces handling folders and makes Explorer itself unusable, but other programs' open/save dialogs will still be Explorer, as those are not possible to replace on Windows through any hacks.