An easy and free program to resize can be found here.
Or, determine the ratio of size increments. If, for example, M has CO of 125, L has CO of 150, and XL has CO of 175....the diff each sz is 25 sts. Increase accordingly to the desired size. If you're good with a calculator you can refig incs/decs/shaping...not all that diff to do. However, the KnitComp program can do that for you.
Also, you can map out any armhole, neckline slopes for the new size using knitters graph paper (free online).