Noob needs Help with Heltec Lora V1 Board

Just found the OpenMQTTGateway project yesterday. And wow thats all what I need…
So I started looking around and surprise surprise I found that I have 2 Heltec Boards with LoRA laying around.

So my next investigation was to find the version of my Heltec board HTIT-WB32LA. Ok. Should be V1.

On site i found entry "heltec-wifi-lora-32. Fine seems what I need.
So I started flashing.

Everything works fine. The AP starts as expected, I could enter WiFi credentials and so on.
After reboot the Board connects to Wifi and my mosquitto server and published some topics.

All fine until … Looking at the entrys with MQTT Explorer

The frequency was not as expected. I would like to use 433MHz. No way, 866MHz should be impossible with my hardware.
So I started to disassemble the board. But as expected I found a SX1278. When looking in the datasheet I found that its max frequency is around 510MHz. So it should be the correct chip for my project.

What to do to change to 433MHz?
Do have to setup platformio and compile it myself?
Do I have to change platformio.ini? Wich entry? Or do I have to change source code?

You can change it through the WebUI or with an MQTT command: