I can't say for sure, but I seem to remember that it gets the information from the same source you get the ADSB information, if that device supplies it. I thought there was a suggestion to note those instruments that require input outside the information given by the ADSB stream, but perhaps it's not been inplemented.
If you've put the instrument in one of your screen layouts, you might try tapping on the instrument. That would seem to me how they would allow you to input a manual temperature, if it's available.