What pattern are you using? Along with what Merigold said, it can be somewhat easy or quite complicated. This site
has a freebie software that allows you to recalc gauge and items. But it works best with patts that don't have complicated st patts or repeats. I've used it when designing or need to scale (diff yarn, ndls) and it's worked very well.
ETA: If you end up making dramatic changes, the easiest way I've found to reconfig things like armholes, necklines, etc is to map out the orig on knitter's graph paper (free online) and then remap it at the new sz/gauge. Takes mere mins and you're guaranteed the shaping is correct. (No fiddling with calculator, etc)