It really depends on if you're speaking of changing it for all items in your account or just selected items, or individual items. It can be done several ways. If you need to change margins for all items to one new formula, you'd do that in your marketplace settings.

1. Changing your default formula -
2. Remove All Override Formulas from All Items -
3. Force Reprice All Items - 

Or you can change the formula for selected items

1. Adding a Custom Formula to Selected Items-
2. Force Reprice the Selected Items - 

Or you can override individual items so that individual items have their own margins/custom formula

1. How to Override an Individual Item -
2.Then Force Reprice the Item