If you have a similar garment with armholes that fit the way you like, take it and use it as a pattern. Turn the one you want to take up inside out and then adjust it accordingly.
Or if you have someone to help you, try on your top and then have someone fit it to you. Imo the way you take it up or in would depend on the amount of adjustment needed primarily and then the way the garment is finished.
You want to keep the same fit through the bustline so you might throw that off if you just took it up at the shoulder seam, depending on the amount of adjustment needed. If it is a lot, I think you'd probably have to take some up at the shoulder and also at the sides.