There are various ways to flash the ESP, if you are an expert feel free to choose whatever way you want. The official way to install the firmware via a browser. To do this open https://install.openbikesensor.org/ with either your Chrome or Edge browser and follow up from there.
For a more none technical description of the process switch to https://www.openbikesensor.org/docs/firmware/.
You can see the old documentation with different variants in the former documentation on github.