Converting your zPIV back to PIV is a simpler process than you might think. Simply, send your zPIV to yourself!
The zPIV spend formular is located in your Wallet's Privacy tab.
The "Pay to" field will be your own address and the "Amount" field will be how much zPIV you will send (max. 7 denoms per transaction).
When you are ready, just click the "Spend Zerocoin" button and you've successfully converted your zPIV!
Keep in mind that if you want to convert more than 90% of your balance back to PIV, you need to disable the automint feature, else it will re-mint 10% of your PIV to zPIV.
Here you'll find a video how to disable the automint feature.