The top can be altered, but if you are not an experienced seamstress you may have difficulties.

Most likely it will need to be taken in at the sides, bustline and at the armholes.

Sometimes if something is way too big, it may have to be taken apart and completely cut down. In that case the alterations may cost more than the top itself.

Try it on and see where it's too big. Often a "size 8" woman can wear a 10 with no alterations at all.

I'd agree that you'd need to try it on. Every company had its own standard of fit. A ten in one company may be an 8 in another. The difference between missy sizes eight and ten is only two inches, at the bust waist and hip. Circumference of the sleeve doesn't vary nearly as much.

A "top" can be anything. There is absolutely no way to tell you how to alter it without seeing the style, and how it fits you.