You are first given the opportunity to select your tariff during onboarding. At any time you can visit 'Settings', then select 'Tariff'. From there you can select your electricity tariff and this will reflect in the cost reporting in the 'Analysis' section. If you have ecoHeat on, your heat pump will also optimise to your tariff, making the most of cheap electricity periods.
For a list of available tariffs, see What tariffs can ecoHeat optimise my heat pump to?