i am completely new to node red and i don't find a perfect way to cut my strings.
I finally managed to get my data per MQTT to node red but know i don't know how to go further.
It looks like the string you have is a valid JSON string. That means you can configure the MQTT In node to output a "parsed JSON Object" - that will convert the string to an object which will make it easier to access the individual properties.
Once you've done that, you'll be able to access the value you want with msg.payload.data.
In the example you've shared, you want to swap EC00 for 00EC. Is that the exact change you always want to make? Or is it a more general requirement to swap the 5/6th characters with the 7/8th bytes?