As described in the New Rule section of the rule sheet, when playing a New Rule it says “If it contradicts a New Rule already in play, discard the old rule.” This is mostly in cases of Draw, Play, and Limit cards, which, of course, directly contradict Rules of the same category. We generally try to include helpful reminders on those cards about what they replace.
All other New Rules… well, most don’t have anything they directly contradict (though there are a few exceptions*) and they are simply added to the collection of New Rules in play, potentially building up to quite a few different things to keep track of. You don’t have to memorize them all – they’re all out there to see – but obviously this can get quite difficult to take in, especially for newer players, which is precisely why we include cards which reduce the number of New Rules in play, like Rules Reset, Trash A New Rule, Let’s Simplify, etc…
*Notable exceptions include the ruling that 1,2,5 and Inflation contradict (and would therefore replace) each other.