You are able to rebalance an individual Pie and you can rebalance your entire portfolio.
To rebalance an individual Pie, go to that Pie and request a rebalance, which will rebalance the selected Pie only.
To rebalance your entire portfolio, request the rebalance when viewing your full portfolio. This will rebalance all Pies within your portfolio.
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.