You can cut an equal amount around the pattern pieces, being careful to add evenly only where it's needed. For instance, if you don't need to add length, don't go beyond the pattern edge at the shoulders or hem. You'll have to move at least some of the markings, again depending on how you need to make it bigger; i.e. if you need to make the bodice bigger, you'll need to move markings for bust line darts. This works if you need to increase by only 1 size, maybe 2. Any more and you'd do better to buy a larger size pattern.
You can increase or make bigger the size of a dress pattern using the grading dress bodices or styles 1 and 2 diagrams. They usually show where and possibly how much to increase the pattern, for each size, for a two-piece front and two-piece back. Visit a tailor or dress maker for more professional advice.