My acoustic guitar has no truss rod and all this time I've been using .12-.53 gauge strings, and until now it's still that gauge, I've bought a .10-.48 but i haven't switched out the strings yet, but i will do soon and i know you need a truss rod when switching out strings what should i do when my guitar doesn't have that?
There is nothing you can do, assuming you stay with concert pitch tuning. Since the new strings will be 10-15% lighter, with less tension involved, there may be some relaxing in the neck.So, nothing will break, the bridge won't be torn off, but you may find that the bow in the neck is too small, and the strings will rattle against the fretwires. Or not. If that happens - and it may take several days to settle, raising of the saddles/bridge will be an option, on a lot of guitars.
Depending on how well the guitar is put together - often reflected in the price - it may cause no problems, except the need to re-intonate, which again, may or may not be an option with that guitar.
Put bluntly, change strings, adjust what you can, if it doesn't succeed, revert to original, put up with it, or put it up for sale, and find another with the crucial parts that will allow such a change.
Firstly, are you 100% sure you have a guitar with a set of 12s on it that has no truss rod? If so either you have an ultra-modern guitar, or more likely a guitar that probably is unhappy with 12s on it in any case.
(Some ultra modern guitars have a high density carbon strip in place of a truss rod, and are designed to not warp at all under string tension. I've heard they can take anything from 13s to 9s and not budge a mm. I don't know if I believe it, but either way this only applies to some pretty expensive new guitars as far as I know)
If it's a conventional wooden neck with no truss rod, I imagine that with 12s the action is probably already quite high anyway, from having too much tension on the neck causing too much bow. I also would imagine if you're going from 12s to 10s then your goal is to make the guitar a lot easier to play: if that's indeed the case, you may find that a decreased neck bow and lower action from putting on 10s is exactly what you want.
In the case that going from 12s to 10s relieves too much tension, you have no set up adjustment options unfortunately. What you can of course do in that case is to simply to buy a set of 11s. With the added effect of reducing the action as well as having lighter strings, you'll probably find they feel "lighter" than you expect anyway.
Before making any final decisions, give the new strings a good few days to settle in, and if they cause an action that's far too low, buy some 11s
I also have an old guitar with no truss rod. My understanding is that the heavier gauge strings require less tension to reach pitch. To understand, and figure out what is best for you. D'Addario has a guitar tension guide for different size scale lengths and different types of strings Google it and you wont be disappointed.