Assuming that you are writing code for a GAP Central and GATT Client device, use the API CyBle_GattcWriteCharacteristicValue() to send a write request to peripheral with whom you are connected.
Refer the project at following location to implement such an application:
Another question: how can this be used to write to multiple peripherals simultaneously?
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To write to multiple peripherals simultaneously, you need to maintain multiple connections simultaneously with these peripherals. Multiple connections is not supported by the device.
Rather, you can use a star network to connect -> send data -> disconnect with one device and then continue it to others. Refer the time multiplexed Central project at following location: