As a default temperature unit Alexa App displayed in Fahrenheit. To change the measurement units we need:
1, Alexa App: Open the Amazon App and choose “settings”
2, Alexa App: Click “Device Settings”
3, Alexa App: Choose “This Device”
4, Alexa App: Choose “Measurement Units”
5, Alexa App: Adjust  “Measurement
Units” to your preference.
Recommendation: After adjustment new settings restart your Alexa app in order to get information updated.