How much smaller do you need to go? What weight of yarn is called for? If you opt for lighter weight, that'd necessitate smaller ndls and you'd automatically net a smaller size. However, if the patt already calls for a lightweight yarn, then you'd need to resize it. I've done this numerous times using a freebie online program here
along with knitters graph paper to remap the neckline and armhole/armscye shaping. One major glitch in totally reworking the numbers is if the orig patt has a specific st patt requiring X number of sts. Then you have to work around whatever that is.