If inside the parameters section of your device there is no option to change from Farenheit to Celcius, then your device is a US device and does not support it.
Either bring it back to the store and complain there, or fix it in a hacky way:
1) Create a virtual value device, eg: celcius
2) Create a blocky rule that looks like:
The top line puts the farenheit in a parameter,
the middle converts it to celcius and puts it in a variable “item” and
the bottom one puts the celcius in a virtual value, so it can be displayed and easily reused.
You can also choose to keep it as a variable if you only need it in blocky.