You can initiate a rebalance for an individual Pie or for the main level of your portfolio.
To rebalance an individual Pie, go to that Pie and click "rebalance." Doing so will rebalance the slices in the selected Pie only.
To rebalance the top level of your portfolio, click "rebalance" while viewing your full portfolio. Keep in mind that rebalancing your portfolio will only bring the Slices at the top level to their target percentages; in other words, if your portfolio is made up of several Pies, the Pies will be brought to their target percentages but the Slices within those Pies will likely remain under or overweight.
Please keep in mind that any time you rebalance a Pie or your portfolio you will generally be initiating a series of buy and sell transactions. Usually every sale of a security has a tax effect which may be a gain or a loss.