Budget allocation

In your campaign dashboard there is a widget called Performance Insights; specifically, of Audiences & related ads. Here you can see how the budget is allocated per audience. 

If you want to adjust your budget per audience (for instance, because one audience group is more profitable than the other) it's possible to do it there. Simply click on the pencil button as shown in the screenshot below.

Note: Bear in mind that it is not possible to lower the budget for an ad set beneath the obligatory limit Facebook requires you to provide.