Re: the mirror's status, assuming that MX Linux and antiX share many of the same repo mirroes,
http://rsync-mxlinux.org/mirmon/packages.html. That one hasn't worked in over a week.
Re: changing mirrors, explore your menu and look for the Repo Manager. I can't tell you exactly where it is as I don't know which window manager you've chosen to use in antiX: it comes with several. Don't forget to run
in a terminal after changing mirrors.
(By the way, it would be helpful to know which version of antiX you downloaded, base, core, net or full. My answer assumes that you're using the full version.)