Look for this line of code in the Peripheral example...
// BLE LED Switch Characteristic - custom 128-bit UUID, read and writable by central
BLEByteCharacteristic switchCharacteristic("19B10001-E8F2-537E-4F6C-D104768A1214", BLERead | BLEWrite);
What properties does it have... it's Read and Write.
You need to add or change this to Notify.
Then when the central device connects it can register to be notified - you'll then need to add in a callback function in the central code, which will trigger when new data arrives.